Transcription coregulator that can have both coactivator and corepressor functions. Isoform 1, but not other isoforms, is involved in the coactivation of nuclear receptors for retinoid X (RXRs) and thyroid hormone (TRs) in a ligand-dependent fashion. In contrast, it does not coactivate nuclear receptors for retinoic acid, vitamin D, progesterone receptor, nor glucocorticoid. Acts as a coactivator for estrogen receptor alpha. Acts as a transcriptional corepressor via its interaction with the NFKB1 NF-kappa-B subunit, possibly by interfering with the transactivation domain of NFKB1. Induces apoptosis in breast cancer cells, but not in other cancer cells, via a caspase-2 mediated pathway that involves mitochondrial membrane permeabilization but does not require other caspases. May also act as an inhibitor of cyclin A- associated kinase. Also acts a component of the CENPA-CAD (nucleosome distal) complex, a complex recruited to centromeres which is involved in assembly of kinetochore proteins, mitotic progression and chromosome segregation. May be involved in incorporation of newly synthesized CENPA into centromeres via its interaction with the CENPA-NAC complex. {ECO:0000269|PubMed:11713274, ECO:0000269|PubMed:12244126, ECO:0000269|PubMed:15082778, ECO:0000269|PubMed:15254226, ECO:0000269|PubMed:16622420}. This is the function of ITGB3BP (integrin subunit beta 3 binding protein, Ensembl gene identifier ENSG00000142856).