Fertility variation is an essential factor limiting efficient production of cattle.There are several factors that influence sperm quality and fertility which can be divided into subcategories of compensatory (sperm viability, motility, etc.), and uncompensatory (molecular defects in the sperm) traits (Dejarnette, 2005).High fertility can be achieved for bulls suffering from compensable sperm defects by increasing the number of spermatozoa deposited in the cow's reproductive tract.Despite providing high numbers of sperm cells with normal morphology (motility and viability), bulls with non-compensable defects may never achieve adequate fertility, and the molecular mechanisms involved in these defects remain unclear.This gap in the knowledge base engenders millions of dollars of economic impact; in spite of this, there is no conventional method to adequately predict sire fertility.A thorough understanding of the mechanisms regulating bull fertility is essential for obtaining consistently high reproductive efficiency, ensuring lower costs and preventing serious timeloss for breeders.Male infertility can be classified as pre-testicular, testicular and post-testicular, depending on its anatomic and physiologic origin.Patrizio and Broomfield have proposed a classification that includes male infertility with a single gene defect, which includes Usher's, Kallmann's and Immotile cilia syndromes; and male infertility with a chromosomal defect including Kleinfelter's, Noonan's and Prader-Will's syndromes as well as deletions on the AZF a, b or c regions of the Y chromosome.(Cram et al., 2001; De Kretser and Baker, 1999;Ferlin et al., 2007;Krausz and Giachini, 2007).Although sperm dysfunction is known to be a major cause of infertility, there is no pharmacological treatment to improve fertility.The only option for subfertile or infertile men is assisted reproductive technology (ART), which usually consists of treatments that might include intrauterine insemination (IUI), in vitro fertilization (IVF), or intracytoplasmic sperm injection (ICSI) depending on the severity of the dysfunction.It has been established that in men undergoing ICSI that 5% of those suffering from oligozoospermia, teratozoospermia and asthenoospermia show an abnormal karyotype compared to 20% of the ones with azoospermia.Our current limited understanding of the cellular and molecular mechanisms involved in sperm function is the main reason for the lack of clinical progress in this area.