A decline in sperm motility is greatly influential on fertilization efficiency. Sperm motility is affected not only by lifestyle and environmental factors but also by genetic factors. However, the genetic influences upon individual differences in sperm motility remain unknown. We have conducted a genome-wide association study (GWAS) in Japanese men, to clarify the genetic determinants of human sperm motility. We identified erb-b2 receptor tyrosine kinase 4 (ERBB4) as a novel locus associated with sperm motility. This is the first genetic evidence for a genome-wide significant association for sperm motility. (By Dr. Youichi Sato, http://jmg.bmj.com/content/early/2018/02/16/jmedgenet-2017-104991 )