Approximately15% of the couples worldwide cannot conceive after one year of regular sexual intercourse, with the male factor accounting for ~40% of all infertility; however, in up to 30% of cases the etiology is unexplained [1,2]. Among these unknown cases of infertility, a genetic etiology for male infertility was long suspected. This was due to cytogenic evidence showing that 0.2% of azoospermic men, who were otherwise phenotypically normal, exhibited Y-chromosome microdeletions . This evidence was supported by karyotyping which revealed autosomal translocations in 1.3% of infertile couples [4-7]. Researchers realized that many cases of male infertility might be genetic because of the failure of most clinical treatments to correct abnormal sperm parameters [8,9]. Observations of sperm counts in various species, including studies of naturally occurring deletions in drosophila via molecular analysis, also led to an intense search for human spermatogenesis gene(s) which might be deficient in some infertile males [10-13]. Recent advancements in molecular methodology have permitted careful mapping of Y-chromosome microdeletions in men with azoospermia and oligozoospermia; in Western populations this frequency varies between 1–35%, depending on inclusion criteria . For Japanese males, a Y-chromosome microdeletion frequency range of 7.6–17% has been reported [15-22]. Such studies have identified three "azoospermic factor" regions (AZF) where deletions occur on the Y-chromosome long arm: AZFa, AZFb, and AZFc. AZFc was shown to contain the most frequently deleted gene cluster, known as the DAZ gene . Several studies found AZFc deletions to be associated with successful retrieval of sperm during testicular sperm extraction (TESE), whereas deletions in AZFa and AZFb were not [23-25]. Histologically, these deletions are associated with various spermatogenetic alterations including Sertoli cell-only syndrome (SCOS), maturation arrest, and hypospermatogenesis.
Recently, several investigators have shown that embryo characteristics following intracytoplasmic sperm injection (ICSI) using sperm obtained from men with Y-chromosome microdeletions were not adversely affected by the deletion [26-31]. The central concern was that vertical transmission of the microdeletion via ICSI might be passed from father to son  or by natural (unassisted) conception [30,33]. Since very few studies concerning Y-chromosome microdeletion have been undertaken in Japanese males (and none in African males), we aimed to investigate the frequency of Y-chromosome microdeletion as well as selected embryo features and reproductive outcome in Japanese and African azoospermic and oligozoospermic men who underwent IVF+ICSI.