We previously isolated the human homeobox gene HOX4A (HOXD3) on chromosome 2 from a human genomic library and determined its nucleotide sequence. In the present study, expression of the HOX4A gene was investigated in human hematopoietic cell lines. Reverse transcriptase-mediated polymerase chain reaction analysis showed that the HOX4A gene was expressed in erythroleukemia HEL and K562 cells but not in promyelocytic leukemia HL-60 cells. To study the role of the HOX4A gene in erythropoiesis, expression vectors containing the HOX4A gene in the sense or antisense orientation were introduced into HEL cells. The sense transfectants overexpressing the HOX4A gene formed aggregates, which were composed of densely associated cells adhering to tissue-culture dishes, whereas the parental HEL cells and antisense transfectants adhered poorly to the dishes. Furthermore, the sense transfectants overexpressing the HOX4A gene attached more efficiently to fibronectin and collagen than did the antisense transfectants and parental HEL cells. Northern blot analysis showed that integrin beta 3 mRNA levels were significantly increased in the HEL cells overexpressing the HOX4A gene, whereas the integrin beta 1 and alpha IIb mRNA levels did not show a distinct correlation with HOX4A mRNA levels. Fluorescence-activated cell sorting analysis showed that the sense transfectants overexpressing the HOX4A gene expressed increased levels of integrin alpha IIb beta 3 (GP IIb-IIIa) complex as compared with the parental HEL cells and antisense transfectants. These results implicate the homeobox gene HOX4A in the regulation of cell adhesion processes.