Epigenetic silencing by the HUSH complex mediates position-effect variegation in human cells

Keeping quiet one gene at a time Chromosomal DNA comes in two flavors—euchromatin, which contains most of the expressed genes, and heterochromatin, which usually remains quiet. But what keeps genes within heterochromatin silent? Tchasovnikarova et al. examined the basis for this type of silencing in mammalian cells (see the Perspective by Brummelkamp). They identified a complex of proteins in human cells they called HUSH that kept particular parts of the genome silent by changing associated histone methylation marks. Science , this issue p. 1481 , see also p. 1433