The rox1 and rox2 RNAs are essential components of the compensasome, whichmediates dosage compensation in Drosophila

The rox1 and rox2 RNAs have been suggested to be components of the dosage c ompensation machinery in Drosophila. We show that both rox RNAs colocalize with the male-specific lethal proteins at hundreds of specific bands along the male X chromosome. The rox RNAs and MSL proteins also colocalize with t he X chromosome in all somatic cells and are expressed in the same temporal pattern throughout development. Genetic analysis shows that the functions of the rox genes are redundant and required for the association of the MSL proteins with the male X chromosome. These data provide compelling evidence for a direct function of the rox RNAs in dosage compensation.