TAT-BH4 (Bcl-xL), primarily localized at the mitochondria, inhibits apoptotic cell death. This compound comprises an N-terminal eosin-labeled cysteine and the protein transduction domain of the HIV TAT protein (amino acids 49 to 57), fused to the Bcl-xL BH4 peptide. It serves as a tool for research into diseases characterized by accelerated apoptosis [1].