The premalignant hematopoietic progenitor cells harboring mutations (e.g., TET2 and DNMT3A) are predisposed to the development of blood cancer, and the acquisition of second-hit mutations (e.g., RHOAG17V and IDH2R172) in a subclone of TFH cells eventually leads to AITL. Here, TET2 is linked to angioimmunoblastic T-cell lymphoma.