miR-21: significantly overexpressed in different tumor types, in particular in bone tumors [63], it is involved in the regulation of many tumor suppressor genes and apoptosis-related proteins including tropomyosin-1 (TPM1), programmed death protein 4 (PDCD4), and metalloproteinase inhibitor 3 (TIMP3) [64]. The gene discussed is PDCD4; the disease is neoplasm.