The cycloidea (cyc) and teosinte branched 1 (tb1) genes code for structurally related proteins implicated in the evolution of key morphological traits. However, the biochemical function of CYC and TB1 proteins remains to be demonstrated. To address this problem, we have analysed the predicted secondary structure of regions conserved between CYC and TB1, and looked for related proteins of known function. One of the conserved regions is predicted to form a non-canonical basic-Helix-Loop-Helix (bHLH) structure. This domain is also found in two rice DNA-binding proteins, PCF1 and PCF2, where it has been shown to be involved in DNA-binding and dimerization. This indicates that the conserved domain most probably defines a new family of transcription factors, which we have termed the TCP family after its first characterised members (TB1, CYC and PCFs). Other plant proteins of unknown function also belong to this family. We have studied two of these in Arabidopsis and have shown that they are expressed in rapidly growing floral primordia. This, together with the proposed involvement of cyc and tb1 in influencing meristem growth, suggests that many members of the TCP family may affect cell division. Some of these genes may have been recruited during plant evolution to generate new morphological traits.