(S (S (BASENP (NN Research) (NN leader) (NNP Inpyo) (NNP Choi)) (VP (VBZ says) (SBAR (S (BASENP (DT the) (NN team)) (VP (VBD observed) (SBAR (IN that) (S (BASENP (NNP VDUP1)) (VP (VP (VBZ plays) (BASENP (DT a) (JJ decisive) (NN role))) (PP (IN in) (S (VP (VBG causing) (S (BASENP (NN stem) (NNS cells)) (VP (TO to) (VP (VB become) (BASENP (NN natural) (NNS killers)))))))))))))))) (PERIOD .))