In recent years, isolation of several genes affecting flower development in Antirrhinum majus made this species a major model system to study this important developmental process. Genes like SQUAMOSA and FLORICAULA are involved in determination of the floral meristem. Their mutation results in the development of bract-forming shoots at positions where normally flowers would develop. The phenotypes obtained upon mutation of the genes found to affect floral organogenesis fall into three major categories. In each category, always the floral organs in two adjacent whorls become homeotically transformed. Based on this observation a simple genetic model has been proposed to explain the establishment of floral organ identity in the four concentric whorls of the flower. The model hypothesizes the independent induction of two developmental pathways specifying floral organ identity after the formation of sepals as the basic type of organ following induction of a floral meristem. One of these pathways is under the control of the PLENA gene, the other is controlled by the DEFICIENS and GLOBOSA genes. These genes, as well as SQUAMOSA, encode transcription factors sharing a conserved DNA binding domain: the MADS-box. In vitro DNA-binding studies complemented with molecular genetic analysis of the respective mutants show that the DEF and GLO proteins may act together in the form of a heterodimer in the regulation of their target genes as well as in autoregulation. The possible interactions between other MADS-box proteins and their role in flower development is under current investigation.