MPL and myeloproliferative neoplasm: Missense mutations are the major type of atypical MPL mutations such as S204P/F, Y591N/D/F, and so on.12, 13 Mutations of MPLW515A, W515S, W515R, and S505N have been demonstrated to be active and functional,14 and S204P/F and Y591N/D/F mutations are weak gain‐of‐function mutations in MPN.