The present invention relates to CLN5 nucleic acid and protein variants and functional derivatives, homologues and analogues thereof, together with cellular expression systems, and uses for same. The present invention also provides genetically modified mammalian animals and methods of use thereof. More particularly, the present invention relates to mammalian animals modified to express a mutation in the CLN5 gene, the expression of which variant CLN5 gene induces or otherwise facilitates the onset or progression of a disease condition, and to methods of use thereof. The animals and variant molecules and cells of the present invention are useful in a wide range of applications including, but not limited to, providing a means of diagnosis and means of screening for proteinaceous and non-proteinaceous molecular modulators for use in therapeutic and/or prophylactic applications.