In KIT D816V+ neoplastic MCs found in the BM of SM patients, and particularly those with advanced SM, KIT is found spontaneously activated, but also other intracellular substrates are aberrantly activated (phosphorylated), such as signal transducer and activator of transcription 5 (STAT5), AKT and extracellular signal-regulated kinases 1/2 (ERK1/2) [31, 32]. Here, MAPK3 is linked to systemic mastocytosis.