Successful experiments involving the production of transgenic mice by pronuclear microinjection are currently limited by low efficiency of random transgene integration into the mouse genome. Furthermore, not all transgenic mice express integrated transgenes, or in other words are effective as functional transgenic mice expressing the desired product of the transgene, thus allowing accomplishment of the ultimate experimental goal--in vivo analysis of the function of the gene or gene network. The purpose of this review is to look at the current state of transgenic technology, utilizing a pronuclear microinjection method as the most accepted way of gene transfer into the mouse genome.