P

P1-derived artificial chromosome (PAC): A vector used to clone DNA fragments (100- to 300-kb insert size; average, 150 kb) in Escherichia coli cells. Based on bacteriophage (a virus) P1 genome. Compare cloning vector.

p53 : Protein mutated in more than 50% of tumors. Normal (not mutated) form of p53 keeps cell from entering cell division cycle. It also has been found to bring about cell death (apoptosis) after DNA damage. 

PAC: (See P1-derived artificial chromosome.)

Paget's disease of the nipple: Rare breast cancer that begins in milk passages (ducts) and spreads to skin of nipple and areola. Affected skin may appear crusted, scaly, red, or oozing. Prognosis is generally better if nipple changes are the only sign of breast disease and no lump can be felt.

Pain specialist, Pain managment specialist: Oncologists, neurologists, anesthesiologists, neurosurgeons, and other doctors, nurses, or pharmacists who are experts in pain managment. A team of health professionals may also be available to address issues of pain control.

Palliative treatment: One that relieves symptoms, such as pain, but is not expected to cure the disease. Main purpose is to improve the patient's quality of life.

Palpation: Using hands to examine. A "palpable" mass is one that can be felt with the fingers or hand..

Pancolitis: Ulcerative colitis that involves the entire colon.

Pancreatectomy: Surgical removal of the pancreas.

Pap test: Some cells are scraped from a woman's cervix and examined by microscope to see if abnormal cells are present. (Also called a "Pap Smear".)

Partial mastectomy: Removal of less than the whole breast, taking only part of the breast in which the cancer occurs and a margin of healthy breast tissue surrounding the tumor.

Pathogen: Substance or micro-organism capable of producing disease.

Pathologist: Doctor specializing in diagnosing and classifying diseases by laboratory tests such as examination of tissue and cells under a microscope. Pathologists determine whether a tumor is benign or cancerous and, if cancerous, the exact cell type and grade.

PCR: See polymerase chain reaction.

Pediatric oncologist: Doctor specializing in cancers of children.

Pediatrician: Doctor specializing in the care of children.

Pelvic examination: An exam of a woman's uterus and other pelvic organs to help find cancers of the reproductive organs. The doctor will visually examine external structures and palpate (feel) the internal organs such as the ovaries and cervix.

Pelvic exenteration: Surgical removal of organs found in the pelvis.

Permanent section: Method of preparing tissue for microscopic examination: Tissue is soaked in formaldehyde, processed in various chemicals, surrounded by a block of wax, sliced very thin, attached to a microscope slide and stained. This usually takes 1-2 days and provides a clear view of the sample so that the presence or absence of cancer can be determined.

PET scan: (See: Positron emission tomography.)

Phage: A virus for which the natural host is a bacterial cell.

Phase I:  Usually refers to U.S. Federal Drug Administration "trials" of new drugs and treatments. Phase-I trials usually involved relatively few human participants and center most on trying to determine the safety of a new drug, device, or procedure.  

Phase II: Once new drugs, devices or procedures have completed Phase I trials satisfactorily (see above), and are proven safe, a Phase II trial is approved and focuses on better determining if a drug or device is effective against a certain disease or condition, like cancer. 

Phase - III trials are usually larger than Phase-I or II trials and seek to fully prove a new drug, device, or procedure's effectiveness, range of application, safety, and other matters of concern to both researchers and the public.  Following successful completion of Phase-I,II,III trials, the FDA may or may not approve the device, procedure or drug for use in treating any or all of conditions for which they were initially developed and tested.  Usually, however, successful completion of Phase III trials eventually leads to FDA approval..

Photodynamic therapy (PDT): Treatment sometimes used for skin cancers, and cancers of esophagus, lung, or bladder. First, nontoxic chemicals are injected into the blood. This is allowed to collect in the tumor for a few days. Then a special type of laser light is focused on the cancer. The light causes the chemical to change so that it can kill cancer cells. The advantage of PDT is that it can kill cancer cells with very little harm to normal cells.

Physical map: Map of the locations of identifiable landmarks on DNA (e.g., restriction enzyme cutting sites, genes), regardless of inheritance. Distance is measured in base pairs. For the human genome, the lowest-resolution physical map is the banding patterns on the 24 different chromosomes; the highest resolution map would be the complete nucleotide sequence of the chromosomes.
Physical therapist: Health professional who uses exercises and other methods to restore or maintain the body's strength, mobility, and function.

Placebo: Inert, inactive substance (a "sugar pill") that may be used in studies (clinical trials) to compare the effects of a given treatment with no treatment. 

Plasmid: Autonomously replicating, extrachromosomal circular DNA molecules, distinct from the normal bacterial genome and nonessential for cell survival under nonselective conditions. Some plasmids are capable of integrating into the host genome. A number of artificially constructed plasmids are used as cloning vectors.

Plastic and reconstructive surgeon: Surgeon specializing in restoring appearance or in reconstruction of removed or injured body parts.

Platelet: Part of the blood that plugs up holes in blood vessels after injury. Chemotherapy can cause a drop in the platelet count, a condition called thrombocytopenia that carries a risk of excessive bleeding.

Pleura: Membrane around the lungs and lining of the chest cavity.

Ploidy: A measurmemt of the amount of DNA in a cell. Ploidy is a "marker" that helps predict how quickly a cancer is likely to spread. Cancers with the same amount of DNA as normal cells are called "diploid" and those with either more or less than that amount are "aneuploid". About two-thirds of breast cancers are aneuploid.

Pnuemonectomy: Surgical removal of a lung.

Polygenic disorder: Genetic disorder resulting from the combined action of alleles of more than one gene (e.g., heart disease, diabetes, and some cancers). Although such disorders are inherited, they depend on the simultaneous presence of several alleles; thus the hereditary patterns are usually more complex than those of singlegene disorders. Compare single gene disorder.

Polymerase chain reaction (PCR): A method for amplifying a DNA base sequence using a heat-stable polymerase and two 20-base primers, one complementary to the (+) strand at one end of the sequence to be amplified and the other complementary to the (-) strand at the other end. Because the newly synthesized DNA strands can subsequently serve as additional templates for the same primer sequences, successive rounds of primer annealing, strand elongation, and dissociation produce rapid and highly specific amplification of the desired sequence. PCR also can be used to detect the existence of the defined sequence in a DNA sample.

Polymerase, DNA or RNA: Enzymes that catalyze the synthesis of nucleic acids on preexisting nucleic acid templates, assembling RNA from ribonucleotides or DNA from deoxyribonucleotides.

Polymorphism: Difference in DNA sequence among individuals. Genetic variations occurring in more than 1% of a population would be considered useful polymorphisms for genetic linkage analysis. Compare mutation.

Polyp: Growth from a mucous membrane commonly found in organs such as the rectum, uterus, and nose.

Polypectomy: Surgical removal of a polyp.

Positional Cloning: a technique used to identify genes, usually those that are associated with diseases, based on their location on a chromosome. This in in contrast to the older, "functional cloning" technique that relies on some knowledge of a gene's protein product. For most diseases, researchers have no such knowledge. For more information, see "Positional Cloning Approach Expedites Gene Hunts" in Human Genome News 6(6).

Positron emission tomography (PET): PET scans create images of the body (or biochemical events) after injection of a low dose of a radioactive form of a substance such as sugar. The scan computes the rate at which the tumor uses the sugar. High-grade tumors generally use more sugar than normal and low-grade tumors. PET scans are especially useful in taking images of the brain, although they are becoming more widely used to find the spread of cancer of the breast, colon, rectum, ovary, or lung. PET scans also are used to see how well tumors are responding to treatment.

Pre-cancerous: (See: Pre malignant.)

Pre-malignant: Changes in cells that sometime become cancer. (Also called precancerous.)

Predisposition: Increased susceptibility to a disease that can be triggered under certain conditions. For example, some women have a family history of uterine cancer and are therefore more likely (but not absolutely destined) to develop uterine cancer.

Prevalence: Measure of the proportion of persons in the population with a certain disease at a given time.

Primary care physician: The doctor a person most likely would normally see first when a problem arises. For example, a primary care physician could be a family practice doctor, a gynecologist, a general practitioner, a pediatrician, or an internal medicine doctor (an internest).

Primary site: Place where cancer begins. Primary cancer is usually named after the organ in which it starts. Cancer that starts in the ovaries, for example, is always called "ovarian cancer" even if it spreads (metastasizes) to other organs such as the kidneys or brain.

Primer: Short preexisting polynucleotide chain to which new deoxyribonucleotides can be added by DNA polymerase.

Probe: Singlestranded DNA or RNA molecules of specific base sequence, labeled either radioactively or immunologically, that are used to detect the complementary base sequence by hybridization.

Progesterone: Female sex hormone released by the ovaries during the menstrual cycle to prepare the uterus for pregnancy and the breasts for milk production (lactation).

Progesterone receptor assay: Lab test done on a sample of breast cancer that shows whether the cancer depends on progesterone for growth. Progesterone and estrogen receptor tests provide more complete information to help in deciding the best cancer treatment for the patient.

Prognosis: Prediction of the course of a disease; the outlook for the treatment or cure of the patient.

Progression: The spread or growth of a disease with or without treatment.

Prokaryote: Cell or organism lacking a membrane-bound, structurally discrete nucleus and other subcellular compartments. Bacteria are prokaryotes. Compare eukaryote. See chromosome.

Promoter: A site on DNA to which RNA polymerase will bind and initiate transcription.

Prophylactic mastectomy: A subcutaneous mastectomy done before any evidence of cancer can be found, for the purpose of preventing cancer. (Sometimes recommended for women at very high risk of breast cancer.)

Prostate: Male gland located just below the bladder and in front of the rectum. The prostate makes a fluid that is part of semen. The tube that carries urine, the urethra, runs through the prostate.

Prostate specific antigen (PSA): A protein made primarily by the prostate. PSA levels may be elevated for a number of benign reasons or due to the presence of prostate cancer. The PSA test is used to help find prostate cancer as well as to monitor treatment.

Prostatectomy : Surgical removal of all or part of the prostate gland.

Prostatitis: Non-cancerous inflammation of the prostate.

Prosthesis: An artificial form to replace a part of the body, such as a breast prosthesis.

Protein: A large molecule composed of one or more chains of amino acids in a specific order; the order is determined by the base sequence of nucleotides in the gene coding for the protein. Proteins are required for the structure, function, and regulation of the bodys cells, tissues, and organs, and each protein has unique functions. Examples are hormones, enzymes, and antibodies.

Protocol: Formal outline or plan, such as a description of what treatments a patient will receive and exactly when each should be given. (See: Regimen.)

PSA: (See: Prostate specific antigen.)

Psychiatrist: Medical doctor specializing in mental health and behavioral disorders. Psychiatrists provide psychological counseling and can prescribe medications.

Psychologist: Health professional who assesses a person's mental and emotional status and provides psyhcological counseling, but cannot prescribe medications. 

Psychosocial: The psychological and/or social aspects of health, disease, treatment, and/or rehabilitation.

Purine: A nitrogencontaining, double-ring, basic compound that occurs in nucleic acids. The purines in DNA and RNA are adenine and guanine.

Pyrimidine: A nitrogen-containing, single-ring, basic compound that occurs in nucleic acids. The pyrimidines in DNA are cytosine and thymine; in RNA, cytosine and uracil.