BCR and chronic myelogenous leukemia, BCR-ABL1 positive: Of note, in 2/6 cases, PMF histological features became clearly evident after CMR of CML with TKI; at the time PMF histology was detected, BCR::ABL1 was negative and JAK2 positive, at a constant allele burden in one case (4) and at an increased allele burden in the other case (9).