Cold sores are lesions around the mouth, lips, nose, and even chin and fingers caused by an infection of the herpes simplex type 1 virus. Over 70% of the population is infected with this virus with outbreaks varying from person to person. These outbreaks are a direct result of the virus remaining in the body and unpredictably causing an immune response (the sores). Although there is no treatment for the virus, there are medications that help treat the cold sores. This reality, however, may soon change.
A recent study published in the Journal of Infectious Diseases identified a 3-megabase region on human chromosome 21 that contains 6 candidate genes associated with cold sores through the use of single nucleotide polymorphism scans. A two-point linkage analysis resulted in a positive correlation between the frequency of cold sore outbreaks and 2 SNP's within the C21orf91 region. The researchers even proposed that C21orf91 be defined as the Cold Sore Susceptibility Gene-1.
C21orf91 in no way increases your chances of being infected with herpes simplex virus 1 in the first place, it is simply correlated with the frequency of cold sore outbreaks. Genetics, according to an infectious disease professor, Dr. Kriesel, are only responsible for 21% of the cold sore outbreaks- the remaining 79% is attributed to environmental factors, the viral strain, and a number of other variables.
Kriesel, John D et al. C21orf91 Genotypes Correlate With Herpes Simplex Labialis (Cold Sore) Frequency: Description of a Cold Sore Susceptibility Gene. J Infect Dis. (2011) 204 (11): 1654-1662. http://jid.oxfordjournals.org/content/204/11/1654.full#cited-by