(S (S (NP (BASENP (DT The) (JJS earliest) (NNS cases)) (PP (IN of) (BASENP (DT the) (NN virus)))) (VP (VP (VBD appeared) (PP (IN in) (BASENP (DT the) (NNP U.S.)))) (PP (IN in) (BASENP (DT the) (JJ late) (NNS 1990s))))) (PERIOD .))