A. Schematic diagram of the assembly procedure. Error correction reduced the rate from 1 error per 1328 bp to 1 error per 9589. X, sites of oligonucleotide errors. Blue arrow, HA or NA coding sequence; gray, plasmid backbone sequence; green arrow, CMV promoter; purple arrow, human pol I promoter; red arrow, murine pol I terminator; brown arrow, pol II terminator; black rectangle, UTR. B. Schematic diagram of the rescue of synthetic influenza viruses from multiple backbones for types A and B influenza strains. PR8x, derivative of a PR8 strain adapted over 5 passages for growth in MDCK cells; Hes, A/Hessen/105/2007 (H1N1); A/CA, A/California/7/2009 (H1N1); Brisbane, B/Brisbane/60/2008 (Victoria lineage); Panama, B/Panama/45/1990 (Yamagata lineage). The photograph of MDCK cells was made by and used with permission from Benjamin Sievers, JCVI.