MADS-box genes play an important role in plant ontogeny, particularly, in the regulation of floral organ induction and development. Eight full-length cDNAs of HAM (Helianthus annuus MADS) genes have been isolated from sunflower. They encode MADS-box transcription factors expressed in inflorescence tissues. In the frames of the ABCDE model, the HAM proteins were classified according to their structural homology to known MADS-box transcription factors. The HAM45 and HAM59 genes encode the homeotic C function and are involved in the control of the identity of pistil and stamens, while the HAM75 and HAM92 genes determine the A identity of floral and inflorescence meristems and petal identity. The HAM31. HAM2, HAM63, and HAM91 genes encode the B function and are involved in the formation of petals and stamens; and the HAM137 gene encodes the E function. Analysis of the expression of MADS-box genes in sunflower has demonstrated that the structural and functional differences between the ray and tubular flowers in the inflorescence could be a consequence of the lack of HAM59 expression during ray flower initiation.