When BCR::ABL1 quantification was compared in 631 paired peripheral blood and bone marrow samples from 283 CML patients, a good overall concordance was observed, but there was a systematic tendency towards higher BCR::ABL1 levels in peripheral blood than in bone marrow, supporting the current practice of using peripheral blood [46]. This evidence concerns the gene BCR and chronic myelogenous leukemia, BCR-ABL1 positive.