mitogens act primarily by influencing a set of proteins which are involved in the restriction of progression through the cell cycle. the g1 checkpoint is controlled most directly by mitogens: further cell cycle progression does not need mitogens to continue. the point where mitogens are no longer needed to move the cell cycle forward is called the "restriction point" and depends on cyclins to be passed. one of the most important of these is tp53, a gene which produces a family of proteins known as p53. it, combined with the ras pathway, downregulate cyclin d1, a cyclin-dependent kinase if they are not stimulated by the presence of mitogens. in the presence of mitogens, sufficient cyclin d1 can be produced. this process cascades onwards, producing other cyclins which stimulate the cell sufficiently to allow cell division. while animals produce internal signals that can drive the cell cycle forward, external mitogens can cause it to progress without these signals.