We report here the cloning of a cDNA encoding a homeobox transcription factor from vascular smooth muscle and describe its unique pattern of mRNA expression at different stages in development. The cDNA isolated is 1576 base pairs in length not including the poly(A) tail and contains an open reading frame coding for a predicted 372-amino acid homeobox protein. During early embryogenesis, expression was detected in the neural tube with a sharp expression boundary occurring at an anterior position, in the myelencephalon, in the third and fourth branchial arches, and in vessels leading from the heart. In adults, however, transcripts were only detected in aortic smooth muscle and lung but were undetectable in cardiac or skeletal muscle, visceral smooth muscle, and many other tissues including brain. In neonates, expression was detected in the outflow tracts of the heart as well as in the cardiomyocytes. The expression pattern of this gene suggests that, although it likely has multiple roles during development, in the adult, it may participate in the control of vascular smooth muscle differentiation and proliferation.