Down-regulated KIT expression in thyroid tumors is in contrast with the over-expression of other tyrosine kinase receptors, such as c-RET and c-MET, or oncogenes, such as c-RAS, indicating that the signaling pathways of different tyrosine kinase receptors can control opposite biological mechanisms, or alternatively affect cell proliferation or differentiation in a specific cell type. Here, NTRK1 is linked to thyroid tumor.