A novel factor featuring a composite AT hook/basic region-leucine zipper DNA-binding domain was isolated from Bonibyx mori follicular cells. Screening of EST databases derived from a variety of metazoans revealed the exclusive presence of BmCbZ homologues in insect species. BmCbZ characteristic features and gene organization are discussed, in comparison to other known bZIP factors. We concordantly propose that this factor establishes a new insect-specific bZIP family. We further present the isolation of the silkmoth homologue of mammalian C/EBP gamma, BmC/EBP gamma, and in vitro evidence for its interaction with BmCbZ. The formation of a BmCbZ-BmC/EBm gamma heterodimer is a prerequisite for binding to specific C/EBP recognition sites on chorion gene promoters, most probably via both major and minor groove interactions. (c) 2005 Elsevier Inc. All rights reserved.