TTC21B and microcephaly: In order to further explore the role of Ttc21b in anterior embryonic development and potentially determine the mechanism leading to the microcephaly in Ttc21baln/aln mutants, we used the Wnt1-Cre and Ap2-Cre alleles to ablate Ttc21b in NCC’s and both NCC’s and surface ectoderm, respectively (Figs 1 and 5).