RB gene Retinal cell RB gene Retinal cell Mutated RB gene inherited from parent Normal cell growth Spontaneous mutation in one copy of RB gene Spontaneous Normal mutation in second Normal cell copy of RB gene cell growth growth Spontaneous Normal Normal mutation in second cell cell copy of RB gene growth growth of growth control Normal Loss cell of growth growth control (a) (创 Copyright 1999 John Wiley and Sons,Inc All rignts reserved RB gene