A cDNA fragment originally identified in U-937 cells as a vitamin D(3)-regulated gene is here designated the glutamate-rich WD-repeat (GRWD1) gene. WD-repeat proteins are a class of functionally divergent molecules that cooperate with other proteins to regulate cellular processes. GRWD1 encodes a 446-amino-acid protein containing a glutamate-rich region followed by four WD repeats. The yeast homologue of GRWD1, Rrb1, has been shown to be an essential protein involved in ribosome biogenesis. Northern analysis of GRWD1 message levels in the myeloid cell line HL-60 undergoing differentiation induced by vitamin D(3) or retinoic acid demonstrate downregulation coincident with slowing of cellular proliferation. A siRNA designed to downregulate GRWD1 similarly results in a decrease in cellular proliferation within 293 cells. Metabolic labeling of cells expressing the siRNA to GRWD1 shows a decrease in global protein synthesis. Finally, nuclear fractionation studies show cosedimentation of GRWD1 with preribosomal complexes, as well as the WD-repeat-containing protein Bop1, which has previously been implicated in ribosome biogenesis. These studies suggest that within mammalian cells GRWD1 plays a role in ribosome biogenesis and during myeloid differentiation its levels are regulated.