Objective        To screen a stable mouse-lung adapted strain of influenza B virus isolated from clinical obtained strains in recent years.       Methods       The influenza B viruses were isolated from clinical patients with influenza B virus only. A mouse-adapted variant of influenza B virus was obtained by 7 sequential mouse-lung passages (inoculated with mannan) and was subsequently cloned by 2 sequential plaque purifications in MDCK cells. Mouse-adapted virus was then confirmed by infection in mice and sequencing for its molecular features.       Results       A stain of mouse-adapted influenza B virus was isolated from 5 clinical strains

and able to infect and kill mice without mannan. Sequencing indicated that there were 2 mutations in hemagglutinin (N212Y and E361K)

and 1 in neuraminidase (S339F). Hemagglutinin E361K was identified as cleavage site.       Conclusion        A mouse-adapted influenza B virus is obtained from adaptation of clinical viruses in mouse

and it might be as a tool for mouse infected model.