AbstractRetinoids and retinoic acid receptors (RARs) are important mediators of normal epithelial cell homeostasis. To assess the role of retinoids and RARs in regulating growth arrest and apoptosis in benign and malignant mammary epithelial cells, two model systems were developed: 1) RAR function was suppressed in retinoid‐sensitive normal human mammary epithelial cells (HMECs) by the dominant‐negative retinoic acid receptor, RARα403 (DNRAR), and 2) retinoid‐resistant MCF‐7 breast cancer cells were transduced with a functional RARβ2. Inhibition of RAR function by the DNRAR in HMECs resulted in retinoid‐resistance, increased proliferation, and dysregulated growth when cells were cultured in reconstituted extracellular matrix (rECM). Expression of RARβ2 in MCF‐7 cells resulted in sensitivity to retinoid‐induced growth arrest and apoptosis. The CREB‐binding protein (CBP) and the homologous protein p300 are tightly regulated, rate‐limiting integrators of diverse signaling pathways and are recruited during retinoid‐mediated transcriptional activation. The relationship between retinoid receptor expression, growth regulation, and transcriptional regulation of CBP/p300 is poorly understood. Inhibition of RAR function in HMECs by DNRAR suppressed expression of CBP/p300 and expression of RARβ2 in MCF‐7 cells promoted induction of CBP/p300 when cells were treated with 1.0 μM all‐trans‐retinoic acid (ATRA). These results suggest that ATRA and RARs regulate growth arrest of HMECs and modulate CBP/p300 protein expression. Since CBP and p300 are normally present in limiting amounts, their regulation by ATRA and RARs may be an important element in the control of transcriptional activation of genes regulating growth arrest and apoptosis. Microsc. Res. Tech. 59:23–40, 2002. © 2002 Wiley‐Liss, Inc.