Variations within the human genome are being studied to determine susceptibility to chronic diseases, as well as infectious diseases. According to Aileen Kenneson and Coleen Boyle, about one sixth of the U.S. population has some degree of hearing loss. Recent research has linked variants in the "gap junction beta 2" ("GJB2") gene to nonsyndromic prelingual sensorineural hearing loss. "GJB2" is a gene encoding for connexin, a protein found in the cochlea. Scientists have found over 90 variants in this gene and sequence variations may account for up to 50% of nonsyndromic hearing loss. Variants in "GJB2" are being used to determine age of onset, as well as severity of hearing loss.