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Morpholino oligos (MOs) are short, antisense oligos made of modified nucleotides. MOs can knock down gene expression by inhibiting mRNA translation, blocking RNA splicing, or inhibiting miRNA activity and maturation. MOs are effective knockdown tools used in developmental biology experiments and RNA-blocking reagents for cells in culture. MOs do not degrade their RNA targets, but instead act via a steric blocking mechanism RNAse H-independent manner. They remain stable in cells and do not induce immune responses. Microinjection of MOs in early Xenopus embryos can suppress gene expression in a targeted manner.

Like all antisense approaches, different MOs can have different efficiencies and may cause off-target, non-specific effects. Often, several MOs need to be tested to find an effective target sequence. Rigorous controls are used to demonstrate specificity, including: