Specific changes in the M1 protein during adaptation of influenza virus to mouse

The matrix (M) gene of influenza virus has been implicated as a determinant of virulence for mouse brain and lung. Comparison of the M gene sequences of the mouse brain adapted variants A/NWS/33 and A/WSN/33 to their parent, A/WS/33, identified two specific amino acid substitutions in the M1 protein which correlated with virulence for mouse: Ala41-->Val and Thr139-->Ala.