Although all three HASs are found in various tumor cell types and both HAS2 and HAS 3 expression is correlated with malignant transformation, HAS2 is the most efficient HA-synthesizing enzyme [30,84]—its expression is fundamental during embryogenesis, as HAS2-deficient mice result in embryonic lethality [85], while HAS1 and HAS3 deletions have only minor effects on the phenotype [81]. Here, HAS1 is linked to neoplasm.