The pCTCON-2 yeast display vector was kindly provided by Prof. Dane Wittrup [23] . The DNA fragments coding the HA1 and HA2 proteins were amplified from plasmid CMV-R-Cali Two Xcm I restriction sites (CCANNNNN/NNNNTGG) were introduced between Nhe I and BamH I sites of pCTCON-2 to create pCTCON-T. Digestion of pCTCON-T by Xcm I restriction nuclease and gel extraction yielded the T-vector with two 39 T overhangs. Random fragments with 39 A tails were inserted between the two Xcm I sites by T-A ligation.