Genetic studies first identified the Polycomb group genes by their defects in transcriptional silencing of the homeotic genes. To identify new Polycomb group genes, we have developed a transgene assay using pairing-sensitive silencing of the mini-white reporter gene caused by a Polycomb Group Response Element (PRE) from the Sex combs reduced homeotic gene. Recessive mutations that disrupt silencing are recovered in mitotic clones in heterozygous flies. We have screened about 98% of the genome and isolated mutations in most of the known Polycomb group genes. We have also isolated mutations in several new genes required for silencing. Using a combination of meiotic recombination mapping, deletion mapping, and whole genome sequencing, we have identified the transcription units corresponding to most of these new silencing genes.