Overexpression of miR-31-3p inhibited medullary thyroid carcinoma (MTC) cell proliferation in vitro and in vivo. (A) Overexpression efficiency of miR-31-3p in 2 human MTC cell lines MZ-CRC-1 and TT was examined by quantitative real-time polymerase chain reaction. (B, C) Effects of miR-31-3p overexpression on viability of TT and MZ-CRC-1 cells were evaluated through Cell Counting Kit-8 assay. (D) Colony formation assay was performed to assess the proliferation of TT and MZ-CRC-1 cells treated with miR-31-3p mimics or mimic control. (E) Nude mice (n=20) were injected with TT cells transfected with miR-31-3p or negative control (NC) followed by the detection of tumor volume after 0, 7, 12, 17, 22, 27, and 32 days after injection. (F) The weight of tumor isolated from mice injected with miR-31-3p overexpressed TT cells. (G) Relative miR-31-3p expression of MTC tumor formed by miR-31-3p mimic or its control transfected TT cells.