In addition to various extracellular regulators and their downstream intracellular mediators (for example, protein kinase C, RhoA and RAC1 guanosine triphosphatases),12,13 Bruton's tyrosine kinase (BTK) was recently suggested to be involved in mediating MM cell migration and homing to the BM. The gene discussed is BTK; the disease is Miyoshi myopathy.