NOTCH1 and acute lymphoblastic leukemia: Although NOTCH1 functions as an oncogene critical for progression and LIC activity in T-ALL, primary patient samples with activating NOTCH1 mutations survive only when co-cultured with MS5 stromal cells expressing high levels of DLL140, indicating a continued dependence on signals from cells in their microenvironment to activate NOTCH1 to support T-ALL cell survival.