The latest version of Pro Tools Express is 10.3.4.
As stated on the product and knowledge base pages as well as the interface boxes that include Pro Tools Express, the supported Mac Operating Systems are Lion and Mountain Lion.
See
http://avid.force.com/pkb/articles/d...-3-4-Downloads which is linked from the product pages that include Pro Tools Express.
Owners of Pro Tools Express bundled with Mbox/Fast Track interfaces can upgrade to Pro Tools 11
here, and this upgrade was recently reduced from $499 to $199. (FYI- Pro Tools 11.0.3 adds Mavericks support, Pro Tools 11.3 adds Yosemite support.)
Regards,
JC