The ABC model of flower development was first formulated by George Haughn and Chris Somerville in 1988. It was first used as a model to describe the collection of genetic mechanisms that establish floral organ identity in the Rosids, as exemplified by "Arabidopsis thaliana", and the Asterids, as demonstrated by "Antirrhinum majus". Both species have four verticils (sepals, petals, stamens and carpels), which are defined by the differential expression of a number of homeotic genes present in each verticil. This means that the sepals are solely characterized by the expression of A genes, while the petals are characterized by the co-expression of A and B genes. The B and C genes establish the identity of the stamens and the carpels only require C genes to be active. Type A and C genes are reciprocally antagonistic.