During development, mammalian organisms increase in size until a limit is reached that is mainly determined by the rate and duration of occurrence of cellular divisions increasing total cell number. This process is mostly regulated by an orchestration of the actions of genes participating in pathways that promote or inhibit growth through systemic or local effects. This view of growth control genes and of their effects on the cell cycle has begun emerging from the results of transgenic and gene knockout experiments, which have also re-emphasized the central involvement of some growth factors and hormones in growth signaling, although mechanistic relationships and details about the coordination of growth with patterning, differentiation and morphogenesis continue to remain largely elusive.