Officials have focused on deer mice, common in the high-elevation eastern Sierra Nevada region.

CNN: STORY HIGHLIGHTS

In the United States, the carriers of hantavirus are deer mice, cotton rats, rice rats and white-footed mice, according to the U.S. Centers for Disease Control and Prevention.

CNN: STORY HIGHLIGHTS

The hybrids were bred again with deer mice, and the type of burrows those "backcross" offspring made show the inheritance patterns of the behaviours - and strong hints of how many genetic changes are required to modify it.

BBC: Mouse burrowing 'in their genes'