CDK6 and CDK4 are cyclin-dependent kinases that regulate cell cycle progression. Their role in G1 to S phase transition is well understood and is redundantly performed by both close homologues. Our lab discovered a novel kinase-independent role of CDK6 in regulating transcription that is not shared by CDK4. In this work I focused on discovering the function of CDK6 within the hematopoietic system, particularly in hematopoietic stem cells (HSCs) and leukemic stem cells (LSCs) as well as in erythrocytes. Cdk6-deficient mice do not show pronounced abnormalities in the HSC compartment under homeostatic conditions with the exception of increased numbers of HSCs within the most quiescent stem cell compartment (dormant, d-HSC). Quiescent HSCs are required upon hematopoietic stress induced by bone marrow (BM) transplantation or myelosuppression via 5-Fluorouracil. Under these experimental conditions Cdk6-deficient HSCs display a significantly reduced ability to repopulate the hematopoietic system. The impaired stress-induced hematopoiesis is paralleled by the reduced ability to down-regulate Egr1 a prerequisite for d-HSCs to leave quiescence. Transcriptional profiling supported Egr1 as a central regulated gene upon CDK6 loss in HSCs. Our observations also hold true for leukemia. BCR-ABLp210+-infected BM cells from Cdk6-/- mice fail to induce disease in the periphery despite the presence of leukemic stem cells in the BM of recipient mice. Again Egr1 levels are high in Cdk6-/- LSCs. In line, knock-down of Egr1 in Cdk6-/- BCR-ABLp210+ LSKs significantly enhances their potential to form growth-factor independent colonies. These findings define CDK6 as an important regulator of HSC and LSC activation via the regulation of Egr1. In the second part of this thesis a novel role for CDK6 in stabilizing the cytoskeleton of erythrocytes was unraveled. Despite the largely normal hematopoiesis Cdk6-deficient mice are slightly anemic while harboring an increased number of erythroid cells in the BM. No effects of CDK6 loss were found in proliferation or differentiation or within stress erythropoiesis. In contrast, we found that peripheral blood erythrocytes have a decreased life span. CDK6, but not CDK4 protein levels are present in mature blood erythrocytes - predominantly associated with cell membranes. We also detected an impaired F-Actin formation in Cdk6-/- erythroblasts which likely contributes to the decreased stability of the erythrocyte cytoskeleton and hence the reduced life span. Taken together I here describe novel functions for CDK6 within in the hematopoietic compartment. As CDK4/6 kinase inhibitors are entering the clinics my work is of great significance to understand the biology of CDK6 and its functions as transcriptional regulator to improve therapeutic strategies.