Caspase 4 is a 377 amino acid protein (43 kDa) that belongs to the Ced3/ICE family of cystein proteases. Caspase 4 is an "upstream" caspase with a long prodomain. It undergoes a cleavage upon activation, leading to p20 and p10 subunits which have 67% homology to mature Caspase 1 over the equivalent coding sequences. Caspase 4 is capable of cleaving itself and the p30 Caspase 1 precursor, and of inducing apoptosis in transfected insect or COS cells. In high concentrations, it can also cleave the nuclear DNA enzyme PARP but the biological relevance of this cleavage is unclear. Caspase 4 mRNA is found in most tissues examined with the exception of the brain. Highest expression is found in spleen and lung, with moderate expression in heart and liver. Low expression is observed with skeletal muscle, kidney, and testis.