The ABC genes (APETALA2 [AP2] and perhaps AP1 for A class, AP3 and PISTILLATA [PI] for B class and AGAMOUS [AG] for C class in Arabidopsis) were cloned in the 1990s [47–51], and the ABC model developed into a molecular model. The ABC genes all encode transcription factors of the MADS box family, except for A class gene AP2, which belongs to the AP2/ERF family. Molecular studies confirmed most of the assumptions of the ABC model.