Infertility affects about one in six couples, and male factors account for roughly half of all cases—often because sperm don’t swim well. Researchers from the University of Osaka have uncovered a key component of the “switch” that keeps the movement signal strong, offering a promising new avenue for both diagnosis and treatment. When this switch is absent, sperm slow down, and fertilization fails. By restoring that signal in the lab, the team rescued swimming and achieved healthy births in mice.