The proline-rich FH1 domain of the formin 2 (Fmn2) gene was disrupted with a neomycin resistance (neo) cassette, abolishing FH1 domain binding function. Formin 2, a straight-actin-filament nucleator expressed in neurons of the central nervous system, is required for progression through the metaphase of Meiosis I. Specifically, formin 2 controls spindle migration and asymmetric spindle positioning in mouse oocytes. Homozygous males are viable and fertile. Homozygous females suffer from sub-fertility and pregnancy loss. Those that do give birth have small litters that tend to die from inadequate nursing. These mice exhibit arrest of meiosis, lack of spindle formation, and lack polar body extrusion. The Jackson Laboratory 2024, used with permission.