The RAB3D cDNA encodes a 24-kD protein as determined by in vitro translation. The deduced 219-amino acid sequence shows highest homology to the RAB3 subfamily of low molecular mass GTP-binding proteins. Northern blot analysis of various human hematopoietic cell lines showed a single transcript of 4.1 kb present at variable levels in each cell line, with highest expression in granulocytic lines. RAB3D was also highly expressed in YJ and HL-60 cells in response to retinoic acid treatment.