The most common CALR frameshift mutations in MPN are a 52-bp deletion denoted CALR del52 (type 1) and a 5-bp insertion called CALR ins5 (type 2), but all referenced mutations lead to the replacement of the wild-type C-terminus and KDEL ER-retention motif by a new sequence rich in methionine and positively charged residues2,3. The gene discussed is CALR; the disease is myeloproliferative disorder.