The X chromosome may be as important as the Y in determining male fertility potential. By refining the analysis of the particular recombination abnormalities in infertile men, this study confirmed that there may be decreased chromosomal pairing quality as well as recombination frequencies in men with non-obstructive azoospermia. Documentation of the normal variability in recombination is a prerequisite for the understanding of changes observed in abnormal situations, such as non-disjunction or a chromosome re-arrangement. It appears that G-group as well as sex chromosomes are most susceptible to having no recombination foci and thus are more susceptible to non-disjunction during spermatogenesis. The growing knowledge of the close relationship between germ cells and stem cells, and the successful manipulation of these cells in vitro, has tremendous implications not only for the treatment and cure of male infertility but also for a host of other medical diseases in the future.