( ( ( A ( possible trigger ) ) ( for activation ) ) ( ( is ( phosphorylation ( by ( ( ( cyclin A-Cdk ) , ) ( which ( ( functions ( before ( cyclin B1-Cdk ) ) ) ( in ( the ( cell cycle ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT A) (JJ possible) (NN trigger)) (PP (IN for) (NP (NN activation)))) (VP (VBZ is) (VP (VBN phosphorylation) (PP (IN by) (NP (NP (NNP cyclin) (NNP A-Cdk)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBD functions) (PP (IN before) (NP (NN cyclin) (NNS B1-Cdk))) (PP (IN in) (NP (DT the) (NN cell) (NN cycle)))))))))) (. .)))
A possible trigger for activation is phosphorylation by cyclin A-Cdk, which functions before cyclin B1-Cdk in the cell cycle.
(ROOT (S (NP (NNS Cyclins)) (VP (VB regulate) (NP (DT the) (NN cell) (NN cycle)) (ADVP (RB only)) (SBAR (WHADVP (WRB when)) (S (NP (PRP they)) (VP (VBP are) (ADJP (RB tightly) (VBN bound) (PP (TO to) (NP (NNS cdks)))))))) (. .)))