A nonreceptor tyrosine kinase of the TEC family that is preferentially expressed in hematopoietic cells14,15 including MM plasma cells,16,17 BTK mediates chemotaxis of MM cells toward stromal cell-derived factor-1 (SDF-1),16,17 which is secreted at high levels in the BM. Here, CXCL12 is linked to Miyoshi myopathy.