Some genes suppress tumor formation.
Familial retinoblastoma Multiple tumors in the retinas of both eyes occurring in the first weeks of infancy. Single tumor appears in one eye sometime in early childhood before the retina is fully developed and its mitosis ceases. Then random mutation of the remaining RB locus in any retinal cell removes the inhibition provided by the Rb protein and the affected cell grows into a tumor. So, in this form of the disease, a germline mutation plus a somatic mutation of the second allele leads to the disease. Such a double hit is an exceedingly improbable event and so only rarely will such a tumor occur.