STAT5A and fibrosarcoma: It is well known that the number of MECs and their activity are regulated by many functional genes and signaling pathways, including by Janus kinase 2 (JAK2) [3], signal transducer and activator of transcription 5a (STAT5A) [4], multiple endocrine neoplasia type 1 (MEN1) [5], AKT kinase/mammalian target of rapamycin kinase (AKT/mTOR) [6], and rapidly accelerated fibrosarcoma/MAP kinase-ERK kinase/Mitogen-Activated Protein Kinases (Raf/MEK/MAPK) [7].