Many species rely on chromosome-based systems-such as one X versus two X chromosomes-to trigger the differences between males and females. To do this they must find ways to count the sex chromosomes and to activate the process of dosage compensation, which corrects the imbalance of sex-linked genes. A paper by Carmi, Kopczynski and Meyer on page 168 of this issue brings us closer to understand chromosome counting in the nematode worm Caenorhabditis elegans. The authors have identified an X-linked gene that encodes a protein related to nuclear-hormone receptors. This protein, SEX-1, represses the transcription of xol-1
, the pivotal gene involved in both dosage compensation and sex determination.