Highly differentiated spermatozoa are generated through multiple cellular and molecular processes maintained by Sertoli cells. The cellular events associated with germ cells include proliferation, protein folding and transportation, as well as sequential changes in chromatin and cell organelles. These processes are strictly controlled by the expression of specific genes, including transcription and DNA replication/repair. This complex spermatogenesis is impaired by a mutation such as gene knockout, which leads to a variety of morphological and functional abnormalities found in mature spermatozoa. An overview of spermatogenesis impairment induced by gene knockout is provided in the present review.