The mouse disabled 2 (mDab2) gene is a mouse homologue of the Drosophila disabled gene and is alternatively spliced to form two isoforms, p96 and p67. Although p96 has been known to regulate the RasŐSos G-protein signal transduction pathway by interacting with Grb2, little is known about the biological function of p67. Recent studies have shown that the expression of mDab2 is markedly up-regulated during the retinoic acid (RA)-induced differentiation of F9 cells, suggesting another role for mDab2 in cell differentiation [Cho, Lee and Park (1999) Mol. Cells 9, 179Ő184). In the present study, we first elucidated the biological function of p67 isoform of mDab2 and identified its binding partner. Unlike p96, p67 largely resides in RA-treated F9 cell nuclei. In this system, p67 interacts with mouse androgen-receptor interacting protein 3, termed the mDab2 interacting protein, which acts as a transcriptional co-regulator. By using a fusion protein with a heterologous DNA-binding domain (GAL4), we showed that p67 had an intrinsic transcriptional activation function. These results suggest that mDab2 p67 may function as a transcriptional co-factor for certain complexes of transcriptional regulatory elements involved in the RA-induced differentiation of F9 cells.