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Two cDNAs were isolated whose dimerized products bind specifically to a DNA sequence,
kappa E2, 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 achaete-scute
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.