Clinical case summary of ASM patient's response to second-line TKI treatment. In patients with pure MCL and ASM, treatment with midostaurin can result in a PR as our case illustrates. This patient had a diagnosis of c-KIT–positive ASM with AMN nor any additional myeloid mutations. He was initially treated with midostaurin 100 mg twice a day. His symptoms and MC disease burden improved, as reflected by reduced spleen size, improvement of skin rash, and decreased tryptase levels. After 18 months his symptoms recurred, including fatigue, sweats, worsening rash, and increased splenomegaly and tryptase levels. He had lost his response to midostaurin. He subsequently was enrolled in the PATHFINDER trial and received avapritinib 200 mg once daily initially. As seen in Figure 1, he had an excellent response, achieving a CR within 3 months of treatment, which has been maintained. He experienced the expected side effects of mild periorbital edema, whitening of his hair, and an “ALP flare,” which is seen with initiation of TKI treatment and normalizes in 4 to 6 weeks. As reported in PATHFINDER, patients with pure ASM achieve a deep, fast response despite previous exposure to another TKI—midostaurin, in his case. Should he have an allogenic bone marrow transplant? The current view is that because he has no AMN and only harbors the c-KIT mutation, he should continue on a reduced dose of avapritinib 100  mg once daily. It will be interesting to see if this dose could be reduced or interrupted to see if he maintains a CR in the future. BMT, bone marrow trephine; H&E, hematoxylin and eosin stain; MRI, magnetic resonance imaging.