As proof of principle for this additional approach, we tested RNAi lines against two Drosophila ID genes with reported eye phenotypes: ubiquitin protein ligase 3a (ube3a), the Drosophila ortholog of UBE3A implicated in Angelman syndrome, and daughterless (da), the ortholog of TCF4 implicated in Pitt-Hopkins syndrome. Here, TCF4 is linked to Pitt-Hopkins syndrome.