Because cellular abnormalities of the mutant STHdhQ111/Q111 striatal cells that express the endogenous full-length mutant Htt protein resemble damaged neurons reported in HD patients (Trettel et al., 2000; Pal et al., 2006; Jin et al., 2013; Yin et al., 2016; Okada et al., 2021), we tested the effect of oleuropein on the cytotoxicity of mutant STHdhQ111/Q111 striatal cells. The gene discussed is HTT; the disease is Huntington disease.