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.