FUNCTION OF THE POLYCOMB PROTEIN IS CONSERVED IN MICE AND FLIES

A key aspect of determination - the acquisition and propagation of cel l fates - is the initiation of patterns of selector gene expression an d their maintenance in groups of cells as they divide and develop. In Drosophila, in those groups of cells where particular selector genes m ust remain inactive, it is the Polycomb-Group of genes that keep them silenced. Here we show that M33, a mouse homologue of the Drosophila P olycomb protein, can substitute for Polycomb in transgenic flies, Poly comb protein is thought to join with other Polycomb-Group proteins to build a complex that silences selector genes. Since members of this gr oup of proteins have their homologues in mice, our results suggest tha t the molecular mechanism of cell determination is widely conserved.