Diagrams of the functional domains of Pax6 and of the alterations in the mutant mouse lines used in this study. Pax6Aey18 mutants lack exons 5a and 6 of the paired domain. Pax64Neu and Pax614Neu mutants carry point mutations at adjacent sites in the third helix of the homeodomain which abolish binding of that domain to its classical P3 DNA target sequence. Pax6Sey-Neu lack the transactivation domain. (PD = paired domain; HD = homeodomain; TA = transactivating domain; PAI/RED = N- and C-terminal DNA-binding sub-domains of the PD).