The identification of SERCA (sarco/endoplasmic reticulum calcium ATPase) as a target for modulating gain-of-function NOTCH1 mutations in Notch-dependent cancers has spurred the development of this compound class for cancer therapeutics. Despite the innate toxicity challenge associated with SERCA inhibition, we identified CAD204520, a small molecule with better drug-like properties and reduced off-target Ca2+ toxicity compared with the SERCA inhibitor thapsigargin. In this work, we describe the properties and complex structure of CAD204520 and show that CAD204520 preferentially targets mutated over wild-type NOTCH1 proteins in T cell acute lymphoblastic leukemia (T-ALL) and mantle cell lymphoma (MCL). Uniquely among SERCA inhibitors, CAD204520 suppresses NOTCH1-mutated leukemic cells in a T-ALL xenografted model without causing cardiac toxicity. This study supports the development of SERCA inhibitors for Notch-dependent cancers and extends their application to cases with isolated mutations in the PEST degradation domain of NOTCH1, such as MCL or chronic lymphocytic leukemia (CLL).Keywords: CAD204520; NOTCH1; NOTCH1 mutation; P-type ATPases screening; PEST mutation; SERCA; T cell acute lymphoblastic leukemia (T-ALL); crystal structure; mantle cell lymphoma (MCL); thapsigargin.