TTBK2 and holoprosencephaly: We found that E10.5 Ttbk2sca11/sca11 embryos exhibit morphological phenotypes that are strikingly similar to those that we previously described in embryos homozygous for an ENU-induced null allele of Ttbk2, Ttbk2bby/bby [4] (referred to from this point as Ttbk2null/null), including holoprosencephaly, a pointed midbrain flexure, and randomized heart laterality (Fig 1A).