sentence: Knockout mice for nine of these factors ( as outlined in [ 35 ] ) have been described and the germline disruption of four of these factors causes embryonic lethality ( Numa , Nusap , Tacc3 , Tpx2 ) , two cause no phenotype ( Tacc2 , Astrin ) , and three display fertility defects ( Hmmr , Hurp , Tektin2 ) [ 36 ] .