rises in sperm head depend on CATSPER. (A) Example of a wild-type sperm with two phases of [Ca2+]i changes. (B, C) Recordings from Catsper1 null sperm. None had the fast (1st phase) response within 2 min, but one (C) had a 2nd phase response. The Ca2+ ionophore ionomycin (5 μM) was applied as a control stimulus. (D) A representative recording from a transgenic sperm expressing an EGFP-CATSPER fusion protein in the Catsper1 null background. (E) Percentage of sperm responsive to BSA application in wild-type (WT; n = 18 imaging runs; 9 mice, 109 cells), Catsper1 null (Mut; n = 7; 3 mice, 30 cells), and a Catsper1 mutant rescued with the EGFP-CATSPER transgene (Tg; n = 12; 5 mice, 57 cells). Cells from Tg mice were imaged with Fura-2 and the others were imaged with Fluo-4. BSA was used at a concentration of 5 mg/ml.