MYC and lentivirus infection: Overexpression of miR-421 into C4–2/N-Myc cells by lentivirus infection suppressed ATM expression (Additional file 3: Figure S3) and abrogated the N-Myc-enhanced migration/invasion (Fig. 5b), suggesting that N-Myc-enhanced migration/invasion is mediated, at least partially, by miR-421/ATM pathway.