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It is currently uncertain to what extent genetic transformations of strategic crops (targeting diverse traits) have improved their N use efficiency (NUE), and what the key factors affecting their NUE are. Based on data collected from 130 publications, the effect sizes of genetic transformations and the key factors influencing NUE for three major cereal crops (rice, maize, and wheat), were investigated using a meta-analysis approach. Genetic transformations increased yield, shoot biomass, N uptake efficiency (NUpE), and partial factor productivity of N (PFPN) in the crops, but decreased shoot NUE (SNUE) and grain NUE (GNUE). Transporter genes improved yield and NUE parameters more efficiently, than did the other gene types. The effect sizes for some NUE parameters varied according to crop species and experimental conditions but did not differ between the overexpression and ectopic expression methods. Most effect sizes did not correlate with gene overexpression levels. These results indicate a promising potential of genetic transformations approaches for improving certain NUE parameters. [ABSTRACT FROM AUTHOR]