Sperm were loaded into 0.25-mL straws (IMV) at a concentration of 2.510 6 sperm/straw and frozen in a programmable freezer (Beltron Instruments) at a rate of –38C/min to – 48C, –738C/min until –158C, and –21.48C/min until – 1408C, at which point straws were plunged into liquid nitrogen for storage. Frozen-thawed sex-sorted sperm were used for IVF within 30 min of sorting (as described in Section2.5).