Mutations in thespalt gene ofDrosophila cause ectopic expression ofUltrabithorax andSex combs reduced.

spalt is a gene required for head and tail development in theDrosophila embryo. When mutated it produces homeotic transformations which are restricted in the embryonic cuticle to both ends of the embryo. However analysis of the distribution of its transcripts has shown that it is also expressed in the trunk region of the embruo. Here I show that two homeotic genesUltrabithorax (Ubx) andSex combs reduced (Scr) can be ectopically expressed in the trunk region of embryos lackingspalt gene function. These results suggest thatspalt has a general role in ensuring the correct spatial expression of other homeotic genes and hence could be another member of thePolycomb class of regulatory genes.