An efficient viral vector containing supernatant is the first key issue to address for gene therapy or basic research projects relying on viral gene transfer. When present in a lentiviral vector backbone as a reporter gene, the expression of the enhanced green fluorescent protein (EGFP) shows strong interests to assess the production of lentiviral vector supernatants. The immediate nondestructive visual analysis of EGFP expression helps to predict the quality of the viral vector supernatant while it is produced. It also largely facilitates the quantitative evaluation of the number of viral particles present in the collected supernatants. Although some traps should be considered when analyzing the expression of a lentivirally transduced EGFP expression unit, the EGFP is the most powerful tool to consider when designing new gene transfer and expression strategies.