AIB1/SRC-3 is a broad-specificity transcriptional coregulator which mediates the activating functions of nuclear receptors and other transcription factors. First cloned in the mouse as the p300 and CBP interacting protein p/CIP, it was the third member of the SRC/p160 coregulator family to be characterized. AIB1/SRC-3 is widely expressed in a variety of tissues in myriad physiological systems, with peaks of expression in the thymus, kidney, aorta, spleen, testis and lungs. AIB1/SRC-3 overexpression has been implicated in breast and ovarian cancer, in addition to cancers of the prostate, endometrium, liver and pancreas. SRC-3 has been shown by null deletion in mice to be required for normal growth, puberty, female reproductive function and mammary gland development.