Engrailed is an evolutionary conserved transcription factor. It contains a homeodomain as a DNA binding moiety. In the fly, engrailed is essential for segment polarity. There, secreted Wnt molecules from anterior cells are required for correct engrailed expression of posterior neighbouring cells. Engrailed in turn drives the expression of Hedgehog, a secreted molecule, resulting in a positive feedback loop for the Wnt expression of the anterior cells. In C. elegans the open reading frame C13G5.1 (LG III) encodes a predicted gene product with high identity to the Drosophila protein Engrailed. Since no cDNAs were available from the C. elegans community, we cloned the cDNA encoded by C13G5.1 using RTPCR. The genefinder prediction was largely confirmed. The gene contains a homeodomain encoding hoemeobox. The degree of amino acid identity within engrailed homeodomains of different species is very high. Furthermore, we analyzed the expression pattern of C13G5.1::gfp in transgenic animals (the construct consists of the promoter region, the entire ORF and a C-teminal gfp fusion). C13G5.1::gfp is transiently expressed in the seam cells of embryos. Expression starts from prior to morphogenesis to the three-fold stage. Moreover, the construct is expressed in one pair of sensory neurons in the head and two pairs of neurons in the tail (the identity of all neurons remains to be determined). These expression data were confirmed by antibody staining using specific monoclonal antibody directed against a portion of the Engrailed homeodomain (the antibody is a kind gift of Dr. Nipam Patel, University of Chicago). To elucidate the in vivo function of engrailed in the worm we are studying a mutant with a deletion in the gene generated by EMS mutagenesis. The deletion spans the majority of the ORF, deleting the promoter region as well as the conserved sequence region. Thus the mutant constitutes most likely a null allele. Preliminary analysis of the mutant revealed severe hypodermal defects. Homozygous animals die as embryos with this phenotype being fully penetrant. Finally, the mutants are rescued by the C13G5.1::gfp construct. A more detailed analysis will be presented at the meeting.