Although the targeting strategy was designed to trap splicing of an early Ttbk2 exon (S4A Fig), the homozygous gene trap mice (Ttbk2gt/gt) were viable past weaning, developing variably penetrant hydrocephalus and polycystic kidneys by 6 months of age (S4C and S4D Fig). Here, TTBK2 is linked to Hydrocephalus.