Mutant Phenotype
Studies with mutants suggest that cyclins are positive regulators of cell cycle entry. In yeast, expression of any of the three G1 cyclins triggers cell cycle entry. Since cell cycle progression is related to cell size, mutations in Cyclin D and its homologues show a delay in cell cycle entry and thus, cells with variants in cyclin D have bigger than normal cell size at cell division.
p27-/- knockout phenotype show an overproduction of cells because cyclin D is not inhibited anymore, while p27-/- and cyclin D-/- knockouts develop normally.
Read more about this topic: Cyclin D