Some MADS-domain transcription factors (MTFs) of flowering plants bind as heterotetramers to two adjacent DNA-binding sites. The central protein-DNA complex shown here illustrates this unique multimeric system of paralogous protein interactions. An investigation of the evolutionary origin of this structure has revealed that quartet formation occurs in the flowering plants, ferns, lycophytes, mosses and algae, dating floral-quartet formation to the transition of plants from water to land. This illustration shows the typical structures for these taxonomic groups arranged along a DNA helix, and the Arabidopsis flower shows the characteristic knock-out phenotype of the MADS-box gene family members, AGAMOUS.