An excessive activation of PARP is also frequently observed during cell death, including in neurodegenerative diseases.9,24,25 When we used an in situ PARP activity assay based on the incorporation of biotin-labelled NAD+,26 we found that PARP activity was strongly increased in the photoreceptors of all four Pde6a mutant genotypes. Here, PDE6A is linked to neurodegenerative disease.