Mir-29 regulates Mcl-1 protein expression and apoptosis. Mcl-1 is an anti-apoptotic protein of the Bcl-2 family that is tightly regulated. Recent studies show that microRNAs, such as mir-29, can regulate Mcl-1 expression. In this study, mir-29b was found to be highly expressed in non-malignant cholangiocytes but downregulated in malignant cholangiocarcinoma cells, which is consistent with increased Mcl-1 protein levels in these cells. Enforced mir-29b expression reduced Mcl-1 protein levels in KMCH cells, indicating that mir-29b directly inhibits Mcl-1 expression. This effect was confirmed by luciferase reporter assays, which showed that mir-29b binds to the 3'UTR of Mcl-1 mRNA, reducing its expression. Mir-29b also sensitized cancer cells to TRAIL-induced apoptosis, while inhibiting mir-29b increased Mcl-1 levels and protected cells from TRAIL-induced apoptosis. These findings suggest that mir-29 is an endogenous regulator of Mcl-1 expression and apoptosis. The study also highlights the role of microRNAs in regulating apoptosis, particularly in cancer cells. The results indicate that mir-29b may be a key regulator of Mcl-1 expression and apoptosis in cholangiocarcinoma and other cancers. The study provides new insights into the regulation of Mcl-1 and its role in apoptosis, and suggests that targeting mir-29b could be a potential therapeutic strategy for cancer treatment.Mir-29 regulates Mcl-1 protein expression and apoptosis. Mcl-1 is an anti-apoptotic protein of the Bcl-2 family that is tightly regulated. Recent studies show that microRNAs, such as mir-29, can regulate Mcl-1 expression. In this study, mir-29b was found to be highly expressed in non-malignant cholangiocytes but downregulated in malignant cholangiocarcinoma cells, which is consistent with increased Mcl-1 protein levels in these cells. Enforced mir-29b expression reduced Mcl-1 protein levels in KMCH cells, indicating that mir-29b directly inhibits Mcl-1 expression. This effect was confirmed by luciferase reporter assays, which showed that mir-29b binds to the 3'UTR of Mcl-1 mRNA, reducing its expression. Mir-29b also sensitized cancer cells to TRAIL-induced apoptosis, while inhibiting mir-29b increased Mcl-1 levels and protected cells from TRAIL-induced apoptosis. These findings suggest that mir-29 is an endogenous regulator of Mcl-1 expression and apoptosis. The study also highlights the role of microRNAs in regulating apoptosis, particularly in cancer cells. The results indicate that mir-29b may be a key regulator of Mcl-1 expression and apoptosis in cholangiocarcinoma and other cancers. The study provides new insights into the regulation of Mcl-1 and its role in apoptosis, and suggests that targeting mir-29b could be a potential therapeutic strategy for cancer treatment.