morpholino, also known as a morpholino oligomer and as a phosphorodiamidate morpholino oligomer (PMO), is type of oligomer molecule (colloquially, an oligo) used in molecular biology to modify gene expression. The molecular structure has a backbone of methylenemorpholine rings and phosphorodiamidate linkages. Morpholinos block access of other molecules to small (~25 base) specific sequences of the base-pairing surfaces of ribonucleic acid (RNA). Morpholinos are used as research tools for reverse genetics by knocking down gene function. They are being studied for application in medicine.