Peculiarities of the germline-restricted chromosome of songbirds

In multicellular organisms, all cells of an individual normally contain the same genetic information and cell differentiation is achieved through regulation of gene expression. There are exceptions, however, where, instead of silencing, certain sequences are permanently deleted from the genome. An interesting example of this programmed DNA elimination has been described in songbirds, where a whole chromosome is lost from somatic cells early on in embryo development and is maintained only in the germ cells.