( ( ( ( Two - ) or ) ( three-letter codes ) ) ( ( are preferred ) . ) )
(ROOT (S (NP (NP (CD Two)) (: -) (CC or) (NP (JJ three-letter) (NNS codes))) (VP (VBP are) (VP (VBN preferred))) (. .)))
Two- or three-letter codes are preferred.
(ROOT (S (NP (DT The) (JJ genetic) (NN code)) (VP (VBZ works) (PP (IN in) (NP (NP (NN series)) (PP (IN of) (NP (JJ three-letter) (NNS codes)))))) (. .)))