p66 inhibition causes incomplete terminal differentiation. (A) Photoreceptors from p66(RNAi) produce processes that project toward the brain (8/12 animals), as determined by immunostaining with an anti‐arrestin antibody (VC‐1). p66(RNAi) animals had projections with normal orientations (8/12 animals), projections disoriented away from the brain (2/12 animals), or no projections (2/12 animals). (B) In situ hybridizations to examine the effect of p66 RNAi on expression of other genes marking differentiated PRNs. p66(RNAi) animals had strongly reduced expression of soxB, eye53‐1, and eye53‐2. White arrowheads indicate PRN expressing otxA and these factors. (C) Transverse views from z‐stack confocal projections showing ventral displacement and separation of PRNs with respect to PCCs in p66(RNAi) animals. (D) opsin+ PRNs from regenerating p66(RNAi) animals do not express neoblast marker smedwi‐1. p66(RNAi) animals showed a delay in PRN (opsin+ cells) regeneration; 4 days after head removal, p66(RNAi) animals presented no PRN (8/9 animals). Cartoons show surgeries (red) and enlarged regions (green). Anterior, top except in (C) (D, dorsal; V, ventral). Bars: 75 μm (B), 50 μm (C), or 25 μm (D).