However, as it was known that SIRT1-null mice that survived weaning exhibit severe growth retardation and decreased circulatory levels of free Insulin-like Growth Factor 1 (IGF-1), a known promoter of tumor growth [33], [42]–[44], concerns were raised that systemic adaptations to global SIRT1 deficiency may have masked the true effect of intestinal SIRT1-deficiency and protected these animals from tumor development. This evidence concerns the gene SIRT1 and neoplasm.