Proposed by A.G. Knudson for cases of retinoblastoma. He observed that 40% of U.S cases were caused by a mutation in the germ-line. However, affected parents could have children without the disease, but the unaffected children became parents of children with retinoblastoma. This indicates that one could inherit a mutated germ-line but not display the disease. Knudson observed that the age of onset of retinoblastoma followed 2nd order kinetics, implying that two independent genetic events were necessary. He recognized that this was consistent with a recessive mutation involving a single gene, but requiring bi-allelic mutation. Hereditary cases involve an inherited mutation and a single mutation in the normal allele. Non-hereditary retinoblastoma involves two mutations, one on each allele. Knudson also noted that hereditary cases often developed bilateral tumors and would develop them earlier in life, compared to non-hereditary cases where individuals were only affected by a single tumor.