Retinoblastoma, a rare childhood tumor of the eye, has served as the prototype of hereditary cancer predisposition. As predicted from Knudson's two-mutation hypothesis, mutational change in both alleles of the RB1 gene is a prerequisite for tumor formation. Genetic analysis has shown that in most patients with sporadic unilateral retinoblastoma these two mutations have occurred in somatic cells. Most patients with sporadic bilateral retinoblastoma and patients with familial disease are heterozygous for a predisposing RB1 gene mutation that can be transmitted via the germ line. The spectrum of RB1 mutations that can cause retinoblastoma is heterogeneous and includes structural and epigenetic alterations. Mutations that leave intact some of the functions of the encoded protein can be associated with a milder phenotypical expression (''low penetrance retinoblastoma''). It is a challenge for future research to identify genetic factors that modify the risk of retinoblastoma and second cancers in individual mutation carriers.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.