Systematic comparison of a color reporter gene and drug resistance genes for the determination of retroviral titers.

Retroviral vectors usually contain drug resistance genes, which are used to select for infected cells and to determine the viral titers. The viral titer is referred to as colony-forming units (CFUs). Color reporter genes, such as the lacZ gene and the green fluorescent protein gene (gfp), have been widely used as markers in retroviral vectors. In this report, a simple and rapid method for the determination of retroviral titers has been developed. The number of viral particles capable of forming individual green cells per unit volume is defined as marker-forming units (MFUs). The MFUs determined by using gfp as a marker were found to be proportional to the CFUs obtained by using drug selection for five different drug resistance genes. In addition, after adjusting the time factor, the MFUs are higher than CFUs in viruses released from 30 stable helper cell lines. The lower titers determined by CFUs are likely due to the toxicity on transduced cells.