To examine the significance of ASS1 in endometrial cancer cells, we disrupted the ASS1 gene in HEC1B and AN3CA cells, which showed ASS1 expression among the endometrial cancer cell lines, using the CRISPR/Cas9 system and successfully established ASS1-knockout (ASS1-KO) HEC1B and AN3CA cells (Fig. 1(b) and see Supplementary Fig. S1(a), S1(b), S1(c) and S2(a)). The gene discussed is ASS1; the disease is endometrial cancer.