We previously demonstrated the association between BTK expression and cell surface levels of CXCR4 in primary MM cells and that BTK expression is more profound in CXCR4+ than in CXCR4– MM cells sorted from the same patient sample.16 To further examine the intraclonal heterogeneity of MM cells with regard to these two factors, we analyzed the expression of BTK and CXCR4 in CD138-selected MM cells obtained from 176 paired samples—random-site BM aspirates and computed tomography-guided fine-needle aspirates of focal lesions defined by magnetic resonance imaging from the same patients. The gene discussed is BTK; the disease is Miyoshi myopathy.