In their previous studies the authors demonstrated that nerve growth factor-induced differentiation of neuroblastoma cell lines was closely related with nerve growth factor receptors and amplification of N-myc oncogenes. The authors have also successfully restored the nerve growth factor receptor expression in the cell lines which lacked expression of nerve growth factor receptors. In order to clearify the role of N-myc oncogenes during nerve growth factor-induced differentiation of neuroblastoma cell lines, a new retroviral vector was constructed to express antisense N-myc genes. This new vector contained a puromycin resistant gene, which allowed the authors to express another gene in the cell lines which had already been neomycin resistant.