BACKGROUND: MicroRNAs are a family of 19- to 25-nucleotides noncoding small RNAs 
that primarily function as gene regulators. Aberrant microRNA expression has 
been described for several human malignancies, and this new class of small 
regulatory RNAs has both oncogenic and tumor suppressor functions. Despite this 
knowledge, there is little information regarding microRNAs in plasma especially 
because microRNAs in plasma, if exist, were thought to be digested by RNase. 
Recent studies, however, have revealed that microRNAs exist and escape digestion 
in plasma.
METHODOLOGY/PRINCIPAL FINDINGS: We performed microRNA microaray to obtain 
insight into microRNA deregulation in the plasma of a leukemia patient. We have 
revealed that microRNA-638 (miR-638) is stably present in human plasmas, and 
microRNA-92a (miR-92a) dramatically decreased in the plasmas of acute leukemia 
patients. Especially, the ratio of miR-92a/miR-638 in plasma was very useful for 
distinguishing leukemia patients from healthy body.
CONCLUSIONS/SIGNIFICANCE: The ratio of miR-92a/miR-638 in plasma has strong 
potential for clinical application as a novel biomarker for detection of 
leukemia.