Alfred Knudson, a pediatrician and cancer geneticist, proposed that in order to develop retinoblastoma, two allelic mutations are required to lose functional copies of both the Rb genes to lead to tumorigenicity. Knudson observed that retinoblastoma often developed early in life for younger patients in both eyes, while in some rarer cases retinoblastoma would develop later in life and only be unilateral. This unique development pattern allowed Knudson and several other scientific groups in 1971 to correctly hypothesize that the early development of retinoblastoma was caused by inheritance of one loss of function mutation to an RB germ-line gene followed by a later de novo mutation on its functional Rb gene allele. The more sporadic occurrence of unilateral development of retinoblastoma was hypothesized to develop much later in life due to two de novo mutations that were needed to fully lose tumor suppressor properties. This finding formed the basis of the two-hit hypothesis. In order to verify that the loss of function of tumor suppressor genes causes increased tumorigenicity, interstitial deletion experiments on chromosome 13q14 were conducted to observe the effect of deleting the loci for the Rb gene. This deletion caused increased tumor growth in retinoblastoma, suggesting that loss or inactivation of a tumor suppressor gene can increase tumorigenicity.