Renal dysplasia is a developmental anomaly of the kidneys which is considered to be the primary lesion in juvenile progressive nephropathy, a condition leading to chronic renal failure in young dogs. It is defined as disorganised development of the renal parenchyma due to abnormal differentiation, and pathological lesions include persistent foetal structures, dysplastic tubules and cartilaginous and osseus metaplasia. The aetiology of renal dysplasia in dogs is still unknown, but possible causes are infection with canine herpesvirus, teratogenic substances or intrauterine ureteral obstruction. In some breeds renal dysplasia is a familial or inherited disease. Clinical signs of chronic renal failure, such as polyuria, polydipsia, vomiting, weight loss, anorexia, lethargy and anemia, usually appear before the age of 2 years.

Serum thymidine kinase 1 (TK1) activity is used as a tumor marker in disease monitoring in veterinary and human medicine. TK1, an intracellular enzyme, is involved in a salvage pathway of DNA precursor synthesis. TK1 is used in DNA precursor production by catalyzing the transfer of the gamma-phosphate-group from a phosphate-donor to the 5?- hydroxyl-group of thymidine forming thymidine-monophosphate. Nucleoside monophosphosphates are finally converted into thymidine-triphosphates. TK1 activity significantly rises in the G1 and the S phase of the cell cycle.