A protein that helps control whether a cell lives or dies by blocking a type of cell death called apoptosis. The gene for BCL2 is found on chromosome 18, and transfer of the BCL2 gene to a different chromosome is seen in many B-cell leukemias and lymphomas. This causes the BCL2 protein to be made in larger amounts, which may keep cancer cells from dying. Also called B-cell leukemia/lymphoma 2 protein.