The Drosophila Hox gene, Sex combs reduced (Scr), is required for patterning the first thoracic segment. The Scr transcription unit spans 35 kb, with at least 35kb of upstream cis-regulatory sequences. Transcriptional silencing of Scr in the second and third thoracic segments requires the Polycomb group genes. We have identified several Scr genomic fragments as Polycomb group response elements (PREs).