The BCL2 proto-oncogene, which encodes a protein localized to the inner mitochondrial membrane, plays a crucial role in blocking programmed cell death. This study investigates the distribution of BCL2 protein within various tissues using a monospecific anti-human BCL2 antibody. BCL2 is found in tissues characterized by apoptotic cell turnover, including germinal centers, thymic medulla, hematopoietic lineages, glandular epithelium, complex differentiating epithelium, and long-lived postmitotic cells. In germinal centers, BCL2 is restricted to the follicular mantle and light zone, where it helps maintain plasma cells and memory B cells. In the thymus, BCL2 is present in developing T cells, particularly in the medulla. In hematopoietic lineages, BCL2 is expressed in precursor cells but not in mature cells. In glandular epithelium, BCL2 is found in tissues regulated by hyperplasia or involution, such as the breast, thyroid, and prostate. In complex epithelia, BCL2 is restricted to stem cells and proliferating zones. In the nervous system, BCL2 is localized to neurons. The study suggests that BCL2's function as an anti-apoptotic protein may contribute to the longevity of progenitor and effector cells in these tissues, potentially increasing their susceptibility to malignancy.The BCL2 proto-oncogene, which encodes a protein localized to the inner mitochondrial membrane, plays a crucial role in blocking programmed cell death. This study investigates the distribution of BCL2 protein within various tissues using a monospecific anti-human BCL2 antibody. BCL2 is found in tissues characterized by apoptotic cell turnover, including germinal centers, thymic medulla, hematopoietic lineages, glandular epithelium, complex differentiating epithelium, and long-lived postmitotic cells. In germinal centers, BCL2 is restricted to the follicular mantle and light zone, where it helps maintain plasma cells and memory B cells. In the thymus, BCL2 is present in developing T cells, particularly in the medulla. In hematopoietic lineages, BCL2 is expressed in precursor cells but not in mature cells. In glandular epithelium, BCL2 is found in tissues regulated by hyperplasia or involution, such as the breast, thyroid, and prostate. In complex epithelia, BCL2 is restricted to stem cells and proliferating zones. In the nervous system, BCL2 is localized to neurons. The study suggests that BCL2's function as an anti-apoptotic protein may contribute to the longevity of progenitor and effector cells in these tissues, potentially increasing their susceptibility to malignancy.