The expression of markers of cerebellar granule cell progenitors is impaired in arl13b-deficient embryos. A–I’ Representative images of in situ hybridization illustrate that the three paralogues of atoh1 (atoh1a, 1b and 1c) are expressed in distinct populations of cerebellar granule cell progenitors. atoh1a is expressed in the URL and LRL. Similar expression patterns of atoh1a occur in wild-type embryos (A) and arl13b mutants (B) while its expression is absent from the oral dorsomedial URL (dashed box) in arl13b morphants (C) at 36 hpf and 48 hpf (A’–C’). The expression patterns of atoh1b remained unaffected in arl13b mutants and morphants (D–F’). The expression level of atoh1c was decreased in the URL in arl13b morphants (I and I’) (cb, cerebellum; URL, upper rhombic lip; LRL, lower rhombic lip). A–I’, Scale bar 100 μm.