Two cDNAs were isolated whose dimerized products bind specifically to a DNA sequence, KEY, located in the immunoglobulin kappa chain enhancer. Both cDNAs share a region of extensive identity to the Drosophila daughterless gene and obvious similarity to a segment in three myc proteins, MyoD, and members of the Drosophila achaefe-scufe and twist gene family. The homologous regions have the potential to form two amphipathic helices separated by an intervening loop. Remarkable is the stringent conservation of hydrophobic residues present in both helices. We demonstrate that this new motif plays a crucial role in both dimerization and DNA binding.