The Dorsal-related immunity factor, Dif is a Drosophilid specific transcription factor that likely arose via gene duplication of Dorsal. An alternative splice variant was discovered for Dif based on bioinformatic analysis and conservation of Dorsal B homologs. This isoform, which I will term Dif B, is expressed in whole flies and S2 cells. The structure of Dif B has been characterized as a 987 amino acid protein that is identical to Dif up to the C- terminal end of the rel homology domain where it diverges completely. A hybrid Dif B cDNA was constructed for future use. Exon-specific RNAi suggests a possible inhibitory role for the Dif B isoform