The partial loss of or loss of function of any white blood cell type is a serious health matter and lymphopoiesis is absolutely necessary for life. Mature lymphocytes are a critical part of the immune system that (with the exception of memory B and T cells) have short lives measured in days or weeks and must be continuously generated throughout life by cell division and differentiation from cells such as common lymphoid progenitors (CLPs) in mice. Were this system to fail, the body would be largely undefended from infection.