(S (S (S (BASENP (NNP CCD)) (VP (VBZ is) (VP (ADVP (RB possibly)) (VP (VBN linked) (PP (TO to) (BASENP (NN pesticide) (NN use))))))) (SBAR (IN though) (S (BASENP (JJ several) (NNS studies)) (VP (VBP have) (VP (VP (VBN found) (BASENP (DT no) (JJ common) (JJ environmental) (NNS factors))) (PP (IN between) (NP (BASENP (JJ unrelated) (NNS outbreaks)) (VP (VBN studied))))))))) (PERIOD .))