Hox genes pattern the anteroposterior axis of animals, determining where limbs and other structures form. Usually, Hox genes expressed in posterior regions of developing organisms suppress or downregulate the expression of more anterior genes. But, on p. , Foronda and co-workers report an exception to this posterior prevalence rule: in the Drosophila female genital disc, posteriorly expressed Abdominal-B(Abd-B) maintains Abdominal-A (Abd-A), rather than repressing it as in the embryonic epidermis. Drosophila genitalia derive from the genital disc, which forms from cells of the abdominal segments A8-10. The researchers report that the Abd-B isoforms M and R are expressed in A8 and A9,respectively, and that Caudal, which directs analia formation, is expressed in A10. Additional findings provide a detailed analysis of the unexpected ways in which Hox genes interact with each other and with Distalless, which specifies appendages, to form genitalia.
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