Expression and functional analyses of Emc have demonstrated that it is a prototype for a protein required for multiple processes in development. Initially characterized as a negative regulator of sensory organ development, it was later found to regulate many other developmental processes and cell proliferation. Its ability to block the function of bHLH proteins by forming heterodimers, which are ineffective in DNA binding, accounts for the role of Emc in preventing the acquisition of several cell fates which are under the control of bHLH proteins. However, while maintaining this repressive molecular mechanism, emc also appears to act as a positive regulator of differentiation.