i just read this on a web site

"The latest hereditary issue, which has come up with the Cavalier is SM. Todate there are no tests available to determine, which non-symptomatic dog or bitch is a carrier, and which puppy might have it or not. Only dogs, which show symptoms, can be tested further and an accurate diagnosis made."

http://www.angasstar.com/healthproblems.htm

from the above web site, it is a bit worrig if you cannot test for it. maybe this is an out of date posting or i am reading it wrong in my haste.