embryo biopsy or preimplantation genetic diagnosis (PGD), diagnostic procedure, used in genetic screening, in which a single cell is removed from an embryo two or three days after it has been conceived through in vitro fertilization. At this age the embryo consists of about eight genetically identical cells. The embryo itself is unaffected and continues to grow while the selected cell's genes are replicated using polymerase chain reaction and then studied for genetic defects. The procedure allows an embryo to be tested before it is implanted into the womb when an inheritable disease or a genetic predisposition to a disease is carried by or exhibited in one or both parents.

Embryo biopsy has been used successfully to screen for such diseases as cystic fibrosis and Tay-Sachs as well as for genes that predispose a person to breast or colon cancer. The procedure can also be used to determine the gender of the embryo. Given this ability, X-chromosome-linked diseases that manifest only in males (hemophilia, for example) can be "screened" by implanting only female embryos (which will carry but will not develop the disease). Another variation, used for women over 35 (women past that age being statistically more likely to give birth to babies with Down syndrome and other chromosomal defects) involves examining the sample to make sure it has the correct number of chromosomes. Embryo biopsy has also been used to select a child who is a compatible donor for a sibling with a life-threatening disease such as certain anemias and leukemias.

See also amniocentesis; birth defects; chorionic villus sampling.

biopsy, examination of cells or tissues removed from a living organism. Excised material may be studied in order to diagnose disease or to confirm findings of normality. Preparatory techniques depend on the nature of the tissue and the kind of study intended. Incisions may be made and total or partial lesions removed in the form of wedges or cylindrical pieces, or scrapings of the surface membranes of internal organs may be collected. Needlelike instruments may be used to pierce the tissues and remove soft inner material. Once the tissue specimen has been obtained it is fixed, i.e., membrane proteins and enzymes are stabilized and chemical and histologic analyses are carried out by pathologists. Tumors are routinely biopsied in order to determine whether they are malignant. Fine needle aspiration is a technique more readily used for certain tumors or lesions because it is less expensive and damaging than traditional surgical biopsy.

Procedure in which cells or tissues are removed from a patient and examined. The sample may be obtained from any organ, by any of several methods, including suction through a needle, swabbing, scraping, endoscopy, and cutting out the entire structure or part of it to be tested. Biopsy is a standard step in distinguishing malignant from benign tumours and can provide other information for diagnosis, particularly concerning such organs as the liver or pancreas. Slides of the tissue are prepared and examined by microscope.

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