Homeobox genes are members of a family of transcription factors that regulate tissue development in many different organisms. We set out to identify homeobox genes that might play a role in hematopoiesis. Using degenerate oligonucleotide primers corresponding to conserved homeobox sequences in the polymerase chain reaction (PCR), we cloned from the HL60 promyelocytic cell line a homeobox gene we now designate PRH. We mapped this gene, which was not homologous to any of the previously described homeobox genes, to chromosome 10, a region outside of the known genomic homeobox clusters. Northern blot hybridization indicates that PRH is preferentially expressed in myeloid and liver cells. The structure, chromosomal location, and pattern of expression of PRH indicate that it represents an orphan homeobox gene that may regulate the lineage development of certain hematopoietic cells.