Models of tumorigenesis. Cancer cells can originate from a cell with stem cell properties, such as an HSC, that acquire an altered phenotype through chromosomal aberrations and/or mutations. This leads to an abnormal PcG activity that represses several tumor-suppressor genes promoting cancer development. Another possibility is that the first chromosomal aberration and/or mutation occurs in a committed progenitor cell. In this case, the abnormal PcG activity confers stem cell properties and self-renewal capacity as well as tumorigenic potential.