The first identified CTPS inhibitor cyclopentenyl cytosine (CPEC) showed initially promising anti-tumor activity in human colon carcinoma, pediatric acute lymphocytic leukemia (ALL) as well as in patient-derived pediatric acute non-lymphocytic leukemia (ANLL) cells (Figure 3F) [120,121,122,123]. Here, CTPS1 is linked to colon carcinoma.