In late-stage cancers like anaplastic thyroid carcinomas, the severe deregulation of gene expression results in the formation of metastasis and thus a poor patient prognosis. Carcinoma cell invasioninto surrounding tissue and blood vessels essentially relies on the transition of epithelial cancer cells to cancer cells with mesenchymal properties. In this thesis, the severe downregulation of microRNAs as well as the de novo synthesis of the RNA-binding protein IGF2BP1 were identified as major triggers for the epithelial-to-mesenchymal transition in anaplastic thyroid carcinoma. These post-transcriptional gene regulators modulate mesenchymal gene expression and thereby mesenchymal cell properties. In addition to thyroid cancer-focused studies, the miTRAP approach was developed. MiTRAP allows the reliable identification of regulatory microRNAs for a target RNA of interest in a cellular context of choice.