As part of a survey of the expression of homeobox-containing genes in human hematopoietic cells, we identified a novel gene (PL1) expressed only in cells of the myelomonocytic lineage (Shen et al., Proc. Natl. Acad. Sci, USA 86, 8536, 1989). On Northern gel analysis, major transcripts of 3.0 and 2.2 kb length are observed. Alternatively spliced homeobox-containing cDNAs, corresponding to the major transcripts, have been cloned from two myeloid leukemia cell libraries. The two cDNAs share the homeodomain and 3' flanking region but have unique 5' flanking regions. The longer transcript, would encode a 496 amino acid homeobox-containing protein, while the shorter message would encode a 94 amino acid homeobox-containing protein lacking the extended amino-terminal region. These two transcripts are differentially expressed in human leukemia cell lines. The larger transcript is exclusively expressed in cells with myelomonocytic features, while the smaller transcript is expressed in a variety of hematopoietic cell types. PL mRNA is also detectable in normal human bone marrow by RNAse protection. Neither transcript is expressed in uninduced teratocarcinoma cells or in the adult human tissues surveyed. The homeodomain is identical to the genomic sequence for Hox 1H, a newly identified member of the Hox 1 locus (Acampora et al. Nucl. Acids Res. 17, 10385, 1989). The PL1 gene was localized to chromosome 7 using chromosome specific blots and sublocalized to region pI4-21 by in situ hybridization of chromosomal spreads, confirming its location within the Hox 1 complex.