Homeotic ( Hox ) genes encode transcription factors that confer segmental identity along the anteroposterior axis of the embryo. However the molecular mechanisms underlying Hox -mediated transcription and the differential requirements for specificity in the regulation of the vast number of Hox -target genes remain ill-defined. Here we show that synthetic Sex combs reduced ( Scr ) genes that encode the Scr C terminus containing the homedomain (HD) and YPWM motif (Scr-HD) are functional in vivo. Synthetic Scr-HD peptides can induce ectopic salivary glands in the embryo and homeotic transformations in the adult fly, act as transcriptional activators and repressors during development, and participate in protein-protein interactions. Their transformation capacity was found to be enhanced over their full-length counterpart and mutations known to transform the full-length protein into constitutively active or inactive variants behaved accordingly in the synthetic peptides. Our results show that synthetic Scr -HD genes are sufficient for homeotic function in Drosophila and suggest that the N terminus of Scr has a role in transcriptional potency, rather than specificity. We also demonstrate that synthetic peptides behave largely in a predictable way, by exhibiting Scr -specific phenotypes throughout development, which makes them an important tool for synthetic biology.