1, 25-Dihydroxyvitamin D 3 [1, 25 (OH) 2 D 3], the hormonal ligand for vitamin D 3, is a potent inducer of myeloid-leukemic-cell differentiation. Such cells differentiate exclusively into monocytes/macrophages in response to this ligand. Since 1, 25 (OH) 2 D 3 transduces its hormone signal through the vitamin D 3 receptor (VDR), a ligand-modulated transcription factor and member of the nuclear hormone receptor superfamily, we sought to identify direct VDR target genes induced during this differentiation process. To do so, we applied a modified differential screen with a nascent-RNA purification strategy using biases for immediate-early-response genes induced by 1, 25 (OH) 2 D 3 in the myelomonocytic cell line U937. Using this screen, we had previously identified p21 Waf1/Cip1 as a gene transcriptionally induced by 1, 25 (OH) 2 D 3 and demonstrated that this induction facilitates the differentiation of U937 cells into monocytes/macrophages (24). Here, we describe in detail our differential screen strategy and the identification and isolation of 20 1, 25 (OH) 2 D 3-inducible genes or unknown cDNAs by means of this screen. One gene newly identified as a target of VDR regulation in myeloid cells is the homeobox HoxA10 gene. HoxA10 protein may act as a general regulator of cell growth, since overexpression of HoxA10 facilitated the differentiation of U937 cells into monocytes/macrophages independent of 1, 25 (OH) 2 D 3 and acted to strongly inhibit the growth of the breast cancer cell line MCF-7 by arresting these cells in G 1.