Definitions of some terms commonly used in the stem cell debate:

Adult stem cell -- A cell taken from mature tissue that can renew itself but has a limited ability to transform into specialized cell types.

Assisted reproductive technology -- Fertility treatments that involve a laboratory handling eggs or embryos, such as in vitro fertilization.

Blastocyst -- A preimplantation embryo of 30 to 150 cells.

Cloning -- The creation of an animal or person that derives its genes from a single other individual.

Differentiation -- The process by which early unspecified cells acquire the features of specific cells such as heart tissue, liver or muscle.

DNA -- The material that contains the instructions for making all the parts of the body.

Embryo -- The earliest stage of development from the single cell to implantation in the uterus.

Embryonic stem cell -- a cell from an embryo that has the potential to become a wide variety of specialized cell types.

Gene -- A unit of heredity that is a segment of DNA.

In vitro -- Done outside the body.

In vivo -- Done within the living body.

Nucleus -- The core of a cell that contains the chromosomes.

Stem cells -- Nonspecialized cells that have the capacity to self-renew and to differentiate into more mature cells.

Tissue or cell culture -- Growth of tissue in a laboratory dish for experimental research

Source: Department of Health and Human Services