The appearance of large hyper-lobate and clustering MKs with or without fibrosis as well as the identification of MKs with hybrid features (both small-sized, hypo-lobate and large-sized hyper-lobate forms) in the marrow of patients with a known CML should prompt testing for mutated JAK2/CALR/MPL. This evidence concerns the gene JAK2 and chronic myelogenous leukemia, BCR-ABL1 positive.