NOTCH1 and acute lymphoblastic leukemia: Aberrant activation of NOTCH1 pathway in T-ALL is mostly induced by (1) ligand-independent activation (somatic mutations, indels and large deletions that disrupt the negative regulatory region), or (2) impairment of the proteasomal degradation of intracellular domain of NOTCH1 (truncation of the PEST domain, NOTCH1 mutations in 3′ untranslated region, and FBXW7 mutations) [166,167,168,169,170,171].