Bcl-2 is the prototype for a family of mammalian genes and the proteins they produce. They govern mitochondrial outer membrane permeabilisation (MOMP) and can be either pro-apoptopic (Bax, Bak and Bok among others) or anti-apoptopic (including Bcl-2, Bcl-xL, and Bcl-w, among an assortment of others). There are a total of 25 genes in the Bcl-2 family known to date.