My first pick is for biotinidase deficiency.
For a good overview, go to: http://www.emedicine.com/ped/topic239.htm
I picked this one for the following reasons:
1) it is a routine neonatal screening test in many U.S. states, so is available and will cost less than $200 to get tested
2) heterozygotes (carriers) have reduced level of enzyme activity, so they can get picked up by the test, no DNA sequencing necessary to identify them
3) babies with severe deficiency have severe neurological problems as well as persistent fungal infections if not treated with biotin, which in many (but not all) cases prevents problems
4) an adult heterozygote had persistent vaginal yeast infections that cleared up with taking biotin, a readily available vitamin (see note below, though)
5) Last but certainly not least, Anagnostouli et al. 1999 reported decreased biotin levels in serum and CSF of MS patients compared to controls. I only have seen the abstract, so don't know if biotinidase was evaluated; the authors proposed other causes, such as possible malabsorption in MS patients. Amazing, no one else has since published on this topic.
On a cautionary note to anyone thinking to hedge their bets and take a bunch of biotin without first determining whether they need extra via testing (I certainly thought of doing this myself), there is an older paper on the rat EAE model that shows biotin deficiency actually protected the rats from EAE (Rabin, 1983). I think if the biotin deficiency is severe enough, an animal (or human) loses the ability have any immune response, autoimmune or no, but this is of course not desirable. Conversely, a person with MS taking lots of excess biotin they may not need might not be a great idea either - it is all totally unknown territory.
Partial biotinidase deficiency per this paper below is, I believe, seen in about 1 in 26 people. So it is more common than I thought. There are many types of specific mutations found for this enzyme though, so the genetics gets pretty complicated.
Swango, K. L.; Demirkol, M.; Huner, G.; Pronicka, E.; Sykut-Cegielska, J.; Schulze, A.; Wolf, B. :
Partial biotinidase deficiency is usually due to the D444H mutation in the biotinidase gene. Hum. Genet. 102: 571-575, 1998.
PubMed ID : 9654207