An open question in the field is whether the Kufs disease/CLN4 phenotype is secondary to a loss of function of CSPα/DNAJC5 due to a dominant negative effect of the mutant forms that would be interfering with endogenous CSPα/DNAJC5 (10, 13, 27–29, 31). This evidence concerns the gene DNAJC5 and adult neuronal ceroid lipofuscinosis.