A 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 phosphorodiamidate molecular backbone, which blocks access of other molecules to specific sequences within antisense nucleic acid sequences. Morpholinos block small (~25 base) regions of the base-pairing surfaces of ribonucleic acid (RNA).