D-type cyclins and their cyclin-dependent kinases are not essential for cell cycle progression, according to two independent reports in Cell this week.

The studies reveal the existence of a previously undescribed mechanism for governing cell cycle progression and for initiating proliferation following mitogenic stimulation.

PeterSicinski's group at Harvard Medical School generated triple knockout mutant mice lacking all three D-type cyclins—cyclin D1, D2, and D3—while a team at the Centro Nacional de Investigaciones Oncologicas in Spain headed by Marcos Malumbres generated the complementary deletions of cdk6 and cdk4 single and double mutants. Both sets of mice developed normally until the later stages of gestation, after which they died due to severe anemia, among other abnormalities. These observations suggest that CDK6 is required for expansion of certain differentiated compartments, rather than for proliferation of early hematopoietic precursors, the Spanish team writes in their paper. Sicinski's group attributes it to a...

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