Myalgic encephalomyelitis/chronic fatigue syndrome affects at least 2 million people in the United States. Despite its prevalence, there’s no laboratory test for the disease, and its diagnosis is based on symptoms like exhaustion, unrefreshing sleep, and light sensitivity. For patients with this debilitating condition, getting a diagnosis is often a long and expensive process. Now, a long-awaited biomarker-based test for the mysterious disease could be on the horizon.