In the model plant thale cress (Arabidopsis thaliana), homeotic mutants are categorized as A, B and C. Ideal class A mutants have carpels in the first whorl instead of sepals, and stamens in the second whorl instead of petals. Class B mutants have sepals rather than petals in the second whorl and carpels rather than stamens in the third; and class C mutants have petals instead of stamens in the third whorl and sepals instead of carpels in the fourth.

The existence of defined classes of mutants suggested simple combinatorial models, such as the 'ABC' model of flower development. This proposes three different floral homeotic functions to explain how the different floral organs adopt their unique identities during development. These functions are termed A, B and C, with A specifying sepals in the first floral whorl; A + B, petals in the second; B + C, stamens in the third; and C, carpels in the fourth. Because they had been identified by mutant analysis, it was clear that the homeotic functions are provided by sets of floral homeotic genes. In Arabidopsis there are at least two A-function genes, two B-function genes and one C-function gene, all of which encode transcription factors — proteins that recognize specific DNA motifs and thereby regulate the expression of the genes that contain them.

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