103L, 106I and 103L+106I mutations in the H5N1- NS gene enhance viral tropism for alveolar tissue in mouse lungs. CD-1 mice were infected intransally with 1 × 105 pfu and lungs were collected 3 dpi. The lungs were inflated and fixed with 3.7% formaldehyde and frozen lung sections were stained with anti-PR8 antibody. The bronchioles are indicated by an arrow. a. Mock uninfected lungs b. rPR8-H5N1-NS-(L103 + I106) infected lungs c. rPR8-H5N1-NS-L103F + I106 infected lungs d. rPR8-H5N1-NS-L103 + I106M infected lungs e. rPR8-H5N1-NS-L103F+I106M infected lungs.