The misregulation or misexpression of selector proteins - transcription factors that regulate batteries of target genes during development - can change the identities of cells and tissues, and the shape of whole organisms.

Consequently, the functional domains of selector proteins are highly conserved. Researchers now provide new insights into how conserved peptide motifs in the Drosophila Hox protein Ultrabithorax (Ubx) mediate different aspects of its function. By both activating and repressing transcription, Ubx controls many morphogenetic decisions in fly embryos,including limb formation, which it prevents by repressing Distal-lesstranscription. On p. 5271, Tour and co-workers investigate several conserved motifs within Ubx that are required for its activation and repression functions. They find that the activation function of Ubx is mediated by an N-terminal motif that is conserved between fly and human Hox proteins, and show that its repressive functions are concentration dependent and involve multiple domains, including the conserved YPWM motif. On p. 5261, Hittinger and colleagues report that the conserved QA peptide in Ubx - originally identified as a motif required for limb repression - has different effects in different tissues. In addition, they show that the requirement for QA in limb repression is dose dependent and partly redundant with Abd-A, another Hox protein. Overall, the researchers suggest that the additive and redundant effects of protein motifs in Ubx, and more generally in other selector proteins, might be important in modulating the effects of their evolution.