(S (S (S (BASENP (NNS Theories)) (VP (VBP have) (VP (VP (VP (VBN been) (ADVP (RB around))) (PP (IN since) (BASENP (DT the) (NNS 1940s)))) (SBAR (IN that) (S (NP (BASENP (NN childhood) (NN exposure)) (PP (TO to) (BASENP (NN infection)))) (VP (VBD was) (ADJP (JJ related) (PP (TO to) (NP (BASENP (DT the) (NN development)) (PP (IN of) (BASENP (NN childhood) (NN leukemia)))))))))))) (COLON --) (S (S (NP (BASENP (NN one)) (PRN (COMMA ,) (PP (IN like) (NP (BASENP (DT the) (JJ working) (NN theory)) (PP (IN of) (BASENP (DT the) (NNP UK) (NN study))))) (COMMA ,))) (VP (VBD said) (SBAR (IN that) (S (NP (BASENP (NN lack)) (PP (IN of) (BASENP (JJ immune) (NN challenge)))) (VP (VBD was) (BASENP (DT a) (NN factor))))))) (COMMA ,) (S (BASENP (DT another)) (VP (SBAR (IN that) (S (BASENP (NN leukemia)) (VP (VBD developed) (PP (IN as) (NP (BASENP (DT a) (JJ delayed) (NN result)) (PP (IN of) (NP (BASENP (DT some) (NN type)) (PP (IN of) (BASENP (NN infection)))))))))))))) (PERIOD .))