( They ( also ( ( note ( that ( ( smokers ( who ( carry ( ( the ( defective gene ) ) ( will ( be ( less ( efficient ( at ( ( turning ( the procarcinogens ) ) ( in tobacco ) ) ) ) ) ) ) ) ) ) ) ( ( ( smoke ( into carcinogens ) ) and ) ( ( may ( therefore ( , ( ( in ( addition ( to ( decreased smoking ) ) ) ) , ) ) ) ) ( be ( less ( likely ( to ( develop ( ( lung and ) ( other ( tobacco-related cancers ) ) ) ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (PRP They)) (ADVP (RB also)) (VP (VBP note) (SBAR (IN that) (S (NP (NP (NNS smokers)) (SBAR (WHNP (WP who)) (S (VP (VBP carry) (SBAR (S (NP (DT the) (JJ defective) (NN gene)) (VP (MD will) (VP (VB be) (ADJP (RBR less) (JJ efficient) (PP (IN at) (S (VP (VBG turning) (NP (DT the) (NNS procarcinogens)) (PP (IN in) (NP (NN tobacco))))))))))))))) (VP (VP (VBP smoke) (PP (IN into) (NP (NNS carcinogens)))) (CC and) (VP (MD may) (ADVP (ADVP (RB therefore)) (PRN (, ,) (PP (IN in) (NP (NP (NN addition)) (PP (TO to) (NP (JJ decreased) (NN smoking))))) (, ,))) (VP (VB be) (ADJP (RBR less) (JJ likely) (S (VP (TO to) (VP (VB develop) (NP (NP (NN lung)) (CC and) (NP (JJ other) (JJ tobacco-related) (NNS cancers))))))))))))) (. .)))
They also note that smokers who carry the defective gene will be less efficient at turning the procarcinogens in tobacco smoke into carcinogens and may therefore, in addition to decreased smoking, be less likely to develop lung and other tobacco-related cancers.
(ROOT (S (NP (NP (NN Smoking) (NN tobaccowould)) (ADJP (RBS most) (JJ likely))) (VP (VBP have) (NP (NP (DT a) (JJ negative) (NN impact)) (PP (IN on) (NP (NP (DT the) (NN ability)) (PP (IN of) (NP (DT an) (ADJP (JJ individual) (S (VP (TO to) (VP (VB succeed) (PP (IN at) (NP (JJ physical) (NNS activities))) (SBAR (IN because) (S (NP (PRP it)) (NP (PRP it)))))))) (NNP decreases) (NN stamina) (CC and) (JJ cardiovascular) (NN efficiency))))))) (. .)))
Smoking tobaccowould  most likely have a negative impact on the ability of an individual to succeed at physical activities because it it decreases stamina and cardiovascular efficiency.