Positional cloning efforts of genes mutated in Batten disease and in the Finnish type of variant late infantile neuronal ceroid lipofuscinosis resulted in the identification of two novel genes, CLN3 and CLN5, and corresponding gene products that proved to be residents of lysosomes. Although the clinical phenotype of these NCL subtypes differs in the age of onset, average life span and EEG findings, the major component of material accumulating in patients' lysosomes is subunit c of mitochondrial ATPase in both these diseases. The CLN3 and CLN5 genes show ubiquitous expression patterns and are targeted to lysosomes in vitro, but the observed synaptosomal localization of the CLN3 protein in neurons would suggest some cell specificity in targeting and function of these proteins. So far, 31 different mutations of the CLN3 gene have been described in Batten patients, with one deletion of 1.02 kb accounting for 75% of disease alleles worldwide. Four CLN5 mutations are known, with one premature stop representing the major founder mutation in the isolated Finnish population. Functional studies of the yeast homolog of CLN3 and increased pH in patients' lysosomes would suggest an involvement of this protein in lysosomal pH homeostasis. Knock-out mouse models for CLN3 have been produced and the histopathology bears a close resemblance to human counterparts with characteristic lysosomal accumulations. Both CLN3 and CLN5 mouse models will provide experimental tools to resolve the pathological cascade in these neurodegenerative diseases.