TID1 is a human homolog of the Drosophila tumor suppressor lethal tumerous imaginal discs and encodes two mitochondrial matrix localized splice variants of human Tid1 designated hTid1S and hTid1L. These proteins are the conserved members of the DnaJ family of proteins which act as cochaperons for mitochondrial Hsp70. They contain a conserved tetrahedrical J domain which binds to Hsp70 chaperones and activates their ATPase activity. Expression of hTid1L increases apoptosis induced by DNA damaging agents as mitomycin C and TNF alpha. A J domain mutant of hTid1L can dominantly suppress apoptosis and in sharp contrast the J domain mutant of hTid1S increases apoptosis. Expression of hTid1S and hTid1L affects cytochrome c release from the mitochondria and caspase 3 activation, while activation of caspase 8 is unaffected. It is strongly suggested that these two splice variants exert their anti and pro apoptotic effects through discrete substrates and activities. Hence the relative abundance of these proteins or their substrates may allow the mitochondria to dampen or enhance the apoptotic signals.