Targeted DNA sequencing of the FACS-sorted PF/RF subpopulations revealed an average of 3-fold enrichment of BTK/PLCG2 mutations in the PF versus the RF, demonstrating that the BTK-mutated reservoir is likely resident within the proliferative compartments, where the tumor microenvironment can provide the strongest supportive signals [7, 16, 19, 61]. This evidence concerns the gene BTK and neoplasm.