Testicular cancer seminoma is one of the most common types of cancer among men of reproductive age.Patients with this condition usually present reduced semen quality,even before initiating cancer therapy.However,the underlying mechanisms by which testicular cancer seminoma affects male fertility are largely unknown.The aim of this study was to investigate alterations in the sperm proteome of men with seminoma undergoing sperm banking before starting cancer therapy,in comparison to healthy proven fertile men (control group).A routine semen analysis was conducted before cryopreservation of the samples (n =15 per group).Men with seminoma showed a decrease in sperm motility (P =0.019),total motile count (P =0.001),concentration (P =0.003),and total sperm count (P =0.001).Quantitative proteomic analysis identified 393 differentially expressed proteins between the study groups.Ten proteins involved in spermatogenesis,sperm function,binding of sperm to the oocyte,and fertilization were selected for validation by western blot.We confirmed the underexpression of heat shock-related 70 kDa protein 2 (P =0.041),ubiquinol-cytochrome C reductase core protein 2 (P =0.026),and testis-specific sodium/potassium-transporting ATPase subunit alpha-4 (P =0.016),as well as the overexpression of angiotensin I converting enzyme (P =0.005) in the seminoma group.The altered expression levels of these proteins are associated with spermatogenesis dysfunction,reduced sperm kinematics and motility,failure in capacitation and fertilization.The findings of this study may explain the decrease in the fertilizing ability of men with seminoma before starting cancer therapy.