Abdominal-B acts in three germ cell layers to fulfill this latter function. Abdominal-B is the last in linkage order and the most posterior acting of the linked homeodomain proteins of the bithorax. Regulatory transcripts of Abdominal-B act as repressors, suppressing embryonic ventral epidermal structures in the 8th and 9th segments of the abdomen. Thus ABD-B r and m proteins are critically involved in establishing cell fate in the tail segments of the fly.
Expression is driven by two promoters.
Earlier, in stage 10, both forms are transcribed in epidermis . In stage 11, both forms are found in epidermis and mesoderm. The r protein's designation as regulatory stems from its unique role in segment 14. There it suppresses myogenesis. It is believed that it also represses transcription of the m form.