During spermatogenesis, most of the sperm DNAbinding protein histones are replaced by transition proteins  Oncotarget s209 www.impactjournals.com/oncotarget and then by protamines. Protamines cause the chromatin to condense tightly and thus generate mature sperm. Aniline blue (AB) preferentially binds to histone lysines; thus, AB staining can assess sperm nuclear maturity [27]. In our study, CYP dramatically increased the percentage of immature sperm. Immature sperm produce oxygen radicals, which can induce sperm death and reduce male fertility [28,29]. Conversely, the correctly condensed chromatin of mature sperm can protect against CYPinduced oxidative damage to sperm DNA. In our study, PAL increased the percentage of mature sperm. Thus, our **data** indicate that PAL may promote the fertility of CYPtreated mice through increase in sperm maturity.