Library normalization. cDNAs of high (1%), medium (0.1%), and low (<0.01%) level transcripts were hybridized back to the starting (nonnormalized) libraries and normalized MPMGp609 library. The level of many prevalent transcripts was reduced by twofold to eightfold, and that of lower-level transcripts increases twofold–threefold. OFP2 is a more recent clustering carried out using a different algorithm (Herwig et al. 1999) and has an even higher normalization rate with only 20,883 clone clusters, and clearly performs better at clustering problematic transcripts such as Cathespin L.