These cells proliferate abnormally rapidly, even compared to other cancer cells. Cancer occurs after cells are genetically altered to proliferate rapidly and indefinitely. Thus the infection can proliferate largely unchecked. Following activation with antigen, B cells begin to proliferate rapidly. Microorganisms can no longer proliferate and continue their malignant or pathogenic activities. HaCaT cells are utilized for their high capacity to differentiate and proliferate "in vitro". Nutrient utilization is dramatically altered when cells receive signals to proliferate. As cells proliferate and mature, they undergo partial cornification. These cells further proliferate and branch in response to FGF-10 signaling. Overexpression of HER2 sidesteps these checkpoints, causing cells to proliferate in an uncontrolled fashion. The spores proliferate by mitosis, growing into a haploid organism. With enough p28sis activity, the cells can proliferate without restriction, resulting in cancer. The surrounding stromal cells (fibroblasts) also proliferate. Following thymus entry, progenitors proliferate to generate the ETP population. After activation by antigen, these B cells proliferate.