Regulator of mitochondrial apoptosis and G1 cell cycle in breast cancer. Protects the breast cancer cells against mitochondrial apoptosis and this effect is mediated through the modulation of BCL2 protein family, which involves the positive regulation of anti-apoptotic member BCL2 and the negative regulation of pro-apoptotic members BAD, BAK1 and PUMA. Required for the normal cell cycle progression during G1 in breast cancer cells through the regulation of CCND1 and CDKN1A. Regulates the level of PP2A regulatory subunit B and PP2A phosphatase activity. Interacts with OMP (By similarity). Interacts with LMO2, possibly leading to regulate the transcriptional activity of a DNA-binding complex containing LMO2. Expressed in central nervous system, with high level in pituitary, cerebellum and temporal lobe. Widely expressed in breast cancer cell lines. Belongs to the BEX family. Was named BEX1 by some authors.