Due to the innumerable advantages of using mice as animal models, plenty of strategies to alter mouse genetics have been developed for the purpose of studying the phenotypic outcome of gene mutations. Up until this day, there have been multiple substantial breakthroughs in gene-targeting methods, namely, the utilization of transgene insertion via random integration, gene targeting via homologous recombination, engineered nuclease-mediated targeted mutation, and the Cre-lox approach. Although having facilitated the considerable progress in biomedical research, these techniques are infamously prone to create undesired phenotypes that have been oftentimes mistaken as gene-mutation-related. Hence, being able to identify and avoid these potential artifacts is the most crucial matter in producing transgenic or targeted mutation mouse strains. In this review, we present the fundamentals in generating different mutant mice and the different ways to ensure the credibility of the target gene expression and the observed mutant phenotypes.