Although increasing BDNF expression by means of viral vectors has led to encouraging results, a number of issues still remain to be resolved, as excess expression of the BDNF transgene can have a deleterious effect on neuronal circuits and learning and memory [63], and some of the vectors are toxic per se and can cause tumour formation due to accidental insertional mutagenesis [64]. The gene discussed is BDNF; the disease is neoplasm.