In the first ever sequencing of twin genomes, researchers searched deep into the genetics of multiple sclerosis, coupling DNA sequencing with a panoramic look at the regulation and expression of genes, only to come up empty-handed.

"It's really a tour de force," said Eric Topol, director of the Scripps Translational Science Institute, who was not involved in the research. "It's a fascinating study. It's sobering that they didn't find obvious [genetic factors]."

The study, published this week in Nature, included three pairs of identical twins, two female pairs and one male, in which one had multiple sclerosis (MS) and the other did not. No one knows the cause of MS. Some evidence points to a genetic root of the neurodegenerative disorder: In thirty percent of identical twins where one has MS, so does the other. Changes to the human leukocyte antigen (HLA) region of chromosome 6 has been linked to an increased susceptibility to MS, and 6 genome-wide association studies implicate 12 other loci in the disease.

However, strong evidence also suggests environmental risk factors for MS: In the majority of identical twin cases, only one has MS, and epidemiological studies have suggested month of birth and decreased sunlight exposure can increase risk of developing MS. "It's one of those complicated diseases where there are both genetic effects and environmental effects," said Stephen Kingsmore, senior author on the study and CEO of the National Center for Genome Resources in Santa Fe, New Mexico.