Multiple Sclerosis (MS) is a highly polymorphic disease characterized by different neurologic signs and symptoms. In MS, racial and genetic factors may play an important role in the geographic distribution of this disease. Studies have reported the presence of several protective alleles against the development of autoimmune disorders. In the case of MS, however, they help define MS as a complex disease, and confirm the importance of environmental agents as an independent variable not associated with ethnicity. We carried out an on-site epidemiological study to confirm the absence of MS or NMO among Lacandonians, a pure Amerindian ethnic group in Mexico. We administered a structured interview to 5,372 Lacandonians to assess by family background any clinical data consistent with the presence of a prior demyelinating event. Every participating subject underwent a comprehensive neurological examination by a group of three members of the research team with experience in the diagnosis and treatment of demyelinating disorders to detect clinical signs compatible with a demyelinating disease. We did not find any clinical signs compatible with multiple sclerosis among study participants. 1. Introduction MS is a highly polymorphic disease characterized by diverse neurological signs and symptoms (e.g., optical neuritis, dizziness, disturbances in bladder control, peripheral sensory neuropathies, and/or limb weakness). In most patients (80%) MS has a relapse-remitting pattern, whereas in a minority it can be primary progressive (6%), secondary progressive (10%), or progressive relapsing (4%). MS incidence rates vary significantly depending on geographic location and ethnic origin, ranging from 1？:？500 in Northern Europe to 1？:？100,000 in tropical countries [1]. Individuals migrating from a tropical region to Northern Europe seem to maintain a low risk. Although a predisposition to develop MS may be hereditary, environmental factors display a significant interaction with genetic factors to help determine disease outcome. An effect of shared environment has not been yet proven, and little is known about potential environmental triggers (e.g., viral infections, including Epstein-Barr or Varicella Zoster virus) [2]. Genes associated with a predisposition to develop MS are being actively sought following two main approaches: broad genomic screenings and targeted gene searches. Over 15 research groups have reported linkages of chromosomal regions with MS: chromosomes 6p (on which HLA is located), 5p, 5q, 17q, and 19q [3–5]. However more than half of the human genome has
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