BCR and chronic myelogenous leukemia, BCR-ABL1 positive: Furthermore, analysis of the cellular composition of CML BM revealed that in addition to BCR::ABL1+ driven reduction in CD19+ B cell frequency, CD45.1+CD19+ B cells (CML exposed) were also significantly reduced in frequency (Fig. 1d; Supplementary Fig. 2e, f) and in absolute cell number (Supplementary Fig. S3a).