The CXCL12/CXCR4 pathway has wide-ranging effects on intracellular signalling12 that could influence RT sensitivity.20 However, the results of our timing and duration studies imply that plerixafor is modulating pathways that influence tumour recovery towards the end of and/or early after a course of fractionated RT, rather than intrinsic cellular sensitivity directly. The gene discussed is CXCL12; the disease is neoplasm.