The expression of CXCR4 is dynamically regulated as the PCs move from the peripheral blood milieu to the BM and significantly decreases upon homing to the BM in response to CXCL12, which is elevated in the BM in contrast to the peripheral blood milieu and its levels are significantly increased in the BM of MM patients (161). The gene discussed is CXCL12; the disease is Miyoshi myopathy.