WD40 repeat
The WD40 repeat ( also known as the WD or beta-transducin repeat ) is a short structural motif of approximately 40 amino acids , often terminating in a tryptophan-aspartic acid ( W-D ) dipeptide . 
<triplet> WD40 repeat <subj> structural motif <obj> instance of <triplet> tryptophan <subj> amino acid <obj> instance of <triplet> aspartic acid <subj> amino acid <obj> instance of