In Group 2, the majority of cases (13/20; 65%) showed an opposite growth of BCR::ABL1 and JAK2 as follows: in 4/20 cases, BCR::ABL1 and JAK2 were coexistent at the time of CML and under TKI, BCR::ABL1 decreased whether JAK2 increased (4, 5, 9, 26); in 11/20 cases, only BCR::ABL1 was present at CML diagnosis and JAK2 was either negative (6/11 cases) (18, 25, 48, 52, 53) or not evaluated (5/11 cases) (19, 22, 38, 46); under TKI, BCR::ABL1 decreased with CMR and JAK2 progressively increased. This evidence concerns the gene JAK2 and chronic myelogenous leukemia, BCR-ABL1 positive.