Arabidopsis MADS proteins AP1, AP3, PI, and AG work together to determine flower organ identity. They can interact with each other, but only specific pairs can bind to DNA. AP1 and AG can form homodimers, while AP3 and PI can form a heterodimer. The L region after the MADS box is crucial for choosing the right partner for DNA binding. These findings shed light on how flowers develop according to the ABC genetic model.