Injection of a sperm from a fertile control elicits Ca2+oscillations and activates mouse oocytes (top panel). Sperm from an infertile patient with a history of ICSI failure was unable to elicit Ca2+ oscillations, and was unable to activate mouse oocytes following injection (middle panel), but was able to do so following the co-injection of PLCζ mRNA (bottom panel). Arrows denote time of sperm injection. 1st PB, first polar body; 2nd PB, second polar body. TO-PRO-3 staining (blue) stains chromatin. Asterisk in inset points to the persistence of the human sperm tail in mouse eggs. Scale bar indicates 10 μm. Reproduced from Yoon et al. (2008) with permission.