Induction of terminal differentiation of neoplastic cells offers potential for a novel approach to cancer therapy. One of the agents being investigated for this purpose in preclinical studies is 1,25-dihydroxyvitamin D(3) (1,25D), which can convert myeloid leukemia cells into normal monocyte-like cells, but the molecular mechanisms underlying this process are not fully understood. Here, we report that 1,25D upregulates the expression of hKSR-2, a new member of a small family of proteins that exhibit evolutionarily conserved function of potentiating ras signaling. The upregulation of hKSR-2 is direct, as it occurs in the presence of cycloheximide, and occurs primarily at the transcriptional level, via activation of vitamin D receptor, which acts as a ligand-activated transcription factor. Two VDRE-type motifs identified in the hKSR-2 gene bind VDR-RXR alpha heterodimers present in nuclear extracts of 1,25D-treated HL60 cells, and chromatin immunoprecipitation assays show that these VDRE motifs bind VDR in 1,25D-dependent manner in intact cells, coincident with the recruitment of RNA polymerase II to these motifs. Treatment of the cells with siRNA to hKSR-2 reduced the proportion of the most highly differentiated cells in 1,25D-treated cultures. These results demonstrate that hKSR-2 is a direct target of 1,25D in HL60 cells, and is required for optimal monocytic differentiation.