The retinoblastoma (RB) protein family plays a conserved and inhibitory role in cell cycle progression in higher eukaryotes. In mammals, this family includes, in addition to RB, the related (RBR) proteins p107 and p130, which appear to have both specific and redundant functions compared to those of the prototypical RB protein. Whereas most plant species seem to possess only one RBR gene, a recent study has shown that in maize there are two types of distinctly regulated RBR proteins, RBR1 and RBR3. Expression of RBR3 RNA is controlled by the RBR1-E2F pathway, and it is upregulated upon inhibition of RBR1 activity by the wheat dwarf virus RepA protein in tissue culture, indicating the presence of a specific compensatory mechanism sustaining high pocket protein activity. Database mining and phylogenetic analyses suggest the presence of two distinct RBR genes to be a unique feature of grasses among plants, which might help to explain their recalcitrance to genetic transformation.