Figure 1 shows a heat map of the 25 most abundant microRNAs in 5 human neutrophil donors. Table S1 shows the microRNAs and their expression levels of those expressed in at least 4 out 5 human neutrophil donors. The most abundant microRNA in all samples tested was miR-223, which negatively regulates granulocyte differentiation and fine tunes neutrophil Dabrafenib function. Interestingly, miR-153-2 is within intron 19 of PTRN2, but we were unable to detect this microRNA in our neutrophil samples. This raises the possibility that these microRNAs may be co-ordinately regulated with the genes in which introns they are located. However, microarray expression data is only available for two of these genes, PTRN2 and IGF2. Interestingly, PTRN2 is downregulated by the same conditions that cause upregulation of the intronic microRNA, and IGF2 was not detected in neutrophils. This suggests that post translational regulation of mature microRNA generation may be playing a part in the regulation of protein expression in human neutrophils. Once this mechanism is understood, this might prove a novel avenue for therapeutic manipulation of neutrophil function. MicroRNAs can influence protein expression by changing either translation from existing mRNAs, or by influencing mRNA stability and hence transcript abundance. In order to determine the possible role of microRNAs in regulating the neutrophil transcriptome, we compared our data with a published microarray analysis of transcriptional profile of human neutrophils, which used similar culture conditions and timepoints to our study. By cross referencing changes in gene expression at mRNA level to our microRNA analysis we were able to identify regulated genes which were also predicted targets of regulated microRNAs. Of the transcripts downregulated in this analysis, 83 contained predicted binding sites for at least 2 of the 6 microRNAs upregulated after 4 hours. Neutrophils are short lived, terminally differentiated leukocytes that play a critical role in the destruction of invading bacteria and fungi. Many studies have reported that gene transcription and protein synthesis are key regulators of neutrophil function. MicroRNAs are a recently discovered small RNA species shown to regulate gene expression and may regulate the expression of up to 30% of all genes. Many microRNAs have been linked to apoptosis in a variety of cell types and tumours. We therefore sought to determine the basal expression of microRNAs in freshlyisolated human neutrophils, their regulation over time, and their regulation upon treatment with GMCSF, in order to identify novel regulators of neutrophil functional longevity. We found highly purified human neutrophils express a distinct repertoire of microRNAs, with freshly isolated neutrophils expressing 148 out the 851 human microRNAs in at least 4 out 5 donors on the Agilent Human V3 microRNA microarray. A recent similar array performed on human neutrophils.