Experimental description: Human tissue was obtained from the NICHD Brain and Tissue Bank for Developmental Disorders at the University of Maryland, Baltimore, MD. The role of the NICHD Brain and Tissue Bank is to distribute tissue, and therefore, cannot endorse the studies performed or the interpretation of results. Informed consent for use of the human tissues for research was obtained in writing from all donors or the next of kin. All subjects were defined as normal controls by forensic pathologists at the NICHD Brain and Tissue Bank. No subjects with prolonged agonal state were used. All samples were taken from the frontal part of the superior frontal gyrus: a cortical region approximately corresponding to Brodmann area 9. For all samples similar proportions of grey and white matter was dissected. Total RNA was isolated from the frozen prefrontal cortex tissue using Trizol (Invitrogen, USA) protocol with no modifications. Prior to low molecular weight RNA isolation, total RNA from 20 male individuals aged between 14 and 58 years was combined in equal amounts. Low molecular weight RNA was isolated, ligated to the adapters, amplified, and sequenced following the Small RNA preparation protocol (Illumina, USA) with no modifications(dataset_1). Technical replication was carried out by independent processing of the mixed sample of 20 individuals starting from the low molecular weight RNA isolation step(dataset_2). Biological replication was carried out by isolating and sequencing low molecular weight RNAs from a single 25-year-old individual(dataset_3). These 3 datasets are stored in 3 different folders named dataset_1,dataset_2 and dataset_3,respectively. Solexa sequencing data description: Each solexa data contains 2 columns, the first column is the sequence (36nt), and the second column is the count of this sequence. If certain base of one sequence can not determined for sure, it will represented as one point.