In this study, we investigated the potential involvement of ubiquitin-dependent degradation in the reduced expression of ID1 in human HCC cells following treatment with usenamine A. As depicted in Fig. 8A, ID1 protein level decreased in both HCC cell lines exposed to cycloheximide (the protein synthesis inhibitor) [22] in a time-dependent manner, and usenamine A treatment promoted the downregulation of ID1 protein levels in the presence of cycloheximide, indicating that usenamine A treatment enhanced the degradation of ID1 protein in HCC cell lines. The gene discussed is ID1; the disease is hepatocellular carcinoma.