NRL and retinal degeneration: The total transcription levels (primer pair recognizing both wt and mutant Elovl4) were similar between control and mutant mice, indicating a continuous transcription of both forms of Elovl4. The general reduction of Elovl4 transcript levels in aged R91W;Nrl-/-;Elovl4mut and R91W;Nrl-/- mice was most likely due to the slow intrinsic retinal degeneration in the R91W;Nrl-/- mouse itself [39].