Abstract: The present invention provides methods of purifying RNA from a biological sample comprising two separate aqueous extractions of the organic phase obtained by mixing the biological sample with a first solution comprising guanidinium isothiocyanate and beta-mercaptoethanol and a second solution comprising phenol, chloroform, and isoamyl alcohol. Also provided are methods of sequencing RNA purified from a biological sample and methods of diagnosing Alzheimer's disease and Parkinson's disease in a subject by determining whether a plurality of miRNAs has deregulated expression in biological sample from the subject.