The microRNA Marker is a set of three synthetic single-stranded RNA oligonucleotides 17, 21 and 25 residues long that have free 5´ ends (i.e., no 5´ phosphate groups). These oligonucleotides can be used as size markers on denaturing polyacrylamide gels and Northern blots. The microRNA Marker is best visualized by staining with SYBR-Gold instead of ethidium bromide (Figure 1).