In addition, studies aimed at determining whether RNA persists in other cornified tissues such as the nail could be of great importance for studying of diseases that affect localized body sites, such as cancer, infection, injury and exposures. The discovery of stable mRNAs and miRNAs distant in the hair shaft from the scalp overcomes a formidable obstacle in the application of RNA diagnostics. Currently, RNA studies based on tissue biopsy and phlebotomy are vulnerable to RNA degradation. In the current study, we found that older, more distal regions of hair still contain detectable mRNAs and miRNAs. These findings have several implications. First, the stability of RNA in older portions of hair suggests that RNA may be stable in hair over many months. This attribute differs greatly from other tissues and blood and potentially resolves a problem in storage and transportation of RNA, necessary for RNA-based diagnostic or biomarker studies. Second, temporal changes in gene expression either physiologic or induced by KRX-0401 chemical, drug or disease might be stored co-linearly along the length of the hair shaft. Use of this spatiotemporal pattern of RNA deposition might provide a novel approach to studying the natural course or inciting events of disease. In addition to these characteristics, the continuous replacement of hair and its ease of access are advantages to developing diagnostic approaches based on hair RNA. While possibly ideal for many types of biomedical studies, the use of hair RNA for molecular studies and diagnosis currently has several limitations. At this time, the amount of variation in RNA expression between different individuals, ages, and genetic backgrounds are not known. In addition, differential growth rates of hair in individuals due to differences in genetic background, age, and other factors obscure accurate measurements of time. Recent studies indicate that growth of human hair may vary from 1.3 to 2.2 months per centimeter in different individuals. More accurate measurements might be made possible with the identification of cyclically expressed genes, which could used to normalize differential growth rates. An additional limitation is that since new cells are added to the hair shaft only during active hair growth, it is not yet known what the effects different stages of the hair cycle might have on the pattern and stability of RNA in the hair. In this case, identification of transcripts representative of the final stages of the hair cycle might be required to determine whether retained transcripts reflect specific portions of the hair cycle. Lastly, because the characteristics that contribute to RNA stability in the hair are unknown, it is possible that RNA stability varies in individuals. These unknown aspects may bias the results of RNA detection in the hair. The use of parallel sequencing of small RNAs provides an atlas of residual transcripts in humans of different genders, populations, and hair type.