The aim of this thesis was the identification of regulated microRNAs during monopoiesis and the analysis of its regulatory mechanisms. At first, the cell line U-937 was established as a model for monocyte differentiation. Afterwards microarray analyses were performed to examine the microRNA expression profiles during the monocytes differentiation. Amongst the highly regulated microRNAs, especially the microRNA-cluster miR-221 / -222 was closely examined as an elevated expression of these microRNAs has been particularly described within solid carcinomas. The analysis of the basic expression of miR-221 / -222 in different, mostly hematopoietic cell lines showed an increased expression of miR-221 within the FLT3-ITD associated cell line MV4-11 compared to non-treated CD34+ stem cells. However, subsequent analysis could not verify the regulation of the miR-221 / -222 cluster via the FLT3-ITD signal transduction pathway. With the help of sequential analyses, p27Kip1, an important cell cycle regulator, could be determined as a potential target of the microRNA-Cluster miR-221 / -222. The results of selective LNA Knockdown experiments strengthened the hypothesis that p27Kip1 is a direct target of miR-221 / -222 in the cell line MV4-11.