PYCR2 and cancer: PYCR2 is an essential enzyme in proline biosynthesis and promotes cancer proliferation and progression [106]; however, histamine is derived from the decarboxylation of histidine, which is catalyzed by an enzyme dramatically down-regulated in cancer [107], and histamine-based therapies can lead to cancer cell apoptosis and senescence and prolong survival in tumor-bearing animals [108].