TAT-BH4 ( Bcl-xL ) localized mainly at the mitochondria, prevents apoptotic cell death. TAT-BH4 ( Bcl-xL ) is a fusion peptide that combines the N-terminal cysteine conjugated protein transduction domain of HIV TAT protein ( amino acids 49 to 57) with the Bcl-xL BH4 peptide. TAT-BH4 can be used for research of diseases caused by accelerated apoptosis.