' [ &%.(9-'&&'()* 3+R* '(* &@'9* >2%'L.&'$(4* * I$%* 9'-#<'1'&=7* &@2* %'9`* 9&%"1&"%2* '(* &@2* -$>2<* '9* %2#<.12>* B=* $(2* !MO* D'&@* 5# P* T4[X* #.%&(2%9b=2.%* U&@2* D2')@&2>* .L2%.)2* $,* &@2* ,$"%* !MO9V4* * g$&2* &@.&* @2&2%$)2(2'&=* '(* 1$(&.1&* %.&29* '9*`($D(* &$* '(1%2.92* &@2* %2#%$>"1&'L2* ("-B2%* '(* .* ($(<'(2.%* ,.9@'$(* hW[i7* 9$* &@2* 29&'-.&29* >2%'L2>* B=* &@'9* .##%$.1@* D'<<* B2*  Screening of Random Peptide Library of Hemagglutinin from Pandemic 2009 A(H1N1) Influenza Virus Reveals Unexpected Antigenically Important Regions The antigenic structure of the membrane protein hemagglutinin (HA) from the 2009 A(H1N1) influenza virus was dissected with a high-throughput screening method using complex antisera. The approach involves generating yeast cell libraries displaying a pool of random peptides of controllable lengths on the cell surface, followed by one round of fluorescence-activated cell sorting (FACS) against antisera from mouse, goat and human, respectively. The amino acid residue frequency appearing in the antigenic peptides at both the primary sequence and structural level was determined and used to identify “hot spots” or antigenically important regions. Unexpectedly, different antigenic structures were seen for different antisera. Moreover, five antigenic regions were identified, of which all but one are located in the conserved HA stem region that is responsible for membrane fusion. Our findings are corroborated by several recent studies on cross-neutralizing H1 subtype antibodies that recognize the HA stem region. The antigenic peptides identified may provide clues for creating peptide vaccines with better accessibility to memory B cells and better induction of cross-neutralizing antibodies than the whole HA protein. The scheme used in this study enables a direct mapping of the antigenic regions of viral proteins recognized by antisera, and may be useful for dissecting the antigenic structures of other viral proteins. The 2009 A(H1N1) influenza virus, which is also referred to as the swine-origin influenza virus (S-OIV), caused the first global influenza pandemic in recent decades [1] . Given continuous antigenic drift and reassortment of heterotypic influenza viruses circulating in human and animal reservoirs, global concerns have been raised regarding an increasing threat of an influenza pandemic [2] . Current treatment strategies for influenza-A viruses, such as vaccines and drugs, have not provided broad and lasting protection, partly due to the constantly evolving nature of the viral surface glycoprotein, hemagglutinin (HA) that allows it to avoid host immune attack.