THE sdc-1 gene acts at an early step in the regulatory hierarchy that controls the choice of sexual fate in Caenorhabditis elegans. It functions at a point before the control of sex determination and X-chromosome dosage compensation diverge. Here we report that sdc-1 encodes a protein of 1,203 amino acids containing seven zinc fingers. This protein motif in combination with other genetic and molecular information suggests that sdc-1 is likely to function as an embryonic transcription factor regulating downstream genes involved specifically in the sex determination and dosage compensation pathways, or regulating other genes involved in the coordinate control of both processes. These results enhance our general understanding of sex determination strategies, which are already known to involve transcriptional regulation1 and alternative RNA splicing2,3 in Drosophila melanogaster, DNA rearrangements in Saccharomyces cerevisiae4, and transcriptional regulation in mammals5,6.