( ( When ( ( the ( oxygen level ) ) ( drops ( below ( 60 % ) ) ) ) ) ( , ( ( the respiration ) ( ( process ( of ( ( making ( energy changes ) ) ( into ( fermentation ( in ( a ( cancer cell ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (DT the) (NN oxygen) (NN level)) (VP (VBZ drops) (PP (IN below) (NP (CD 60) (NN %)))))) (, ,) (NP (DT the) (NN respiration)) (VP (VBP process) (PP (IN of) (S (VP (VBG making) (NP (NN energy) (NNS changes)) (PP (IN into) (NP (NP (NN fermentation)) (PP (IN in) (NP (DT a) (NN cancer) (NN cell))))))))) (. .)))
When the oxygen  level drops below 60%, the respiration process of making energy changes into  fermentation in a cancer cell.
(ROOT (S (NP (NNP Fermentation)) (VP (VBZ is) (NP (NP (DT the) (NN process)) (PP (IN of) (S (VP (VBG making) (NP (NN atp)) (PP (IN without) (NP (NN oxygen)))))))) (. .)))