A subsequent increase in the number of FB L4 neurons in the cancer groups may result from either neuronal sprouting of the L4 afferents towards the tumour mass (NGF induced: [37,41,47]) and/or leakage of the FB tracer via cancer-induced perforations in the cortical bone or internal bone compartments, resulting in labelling of additional L4 afferents. The gene discussed is NGF; the disease is cancer.