Table 1.

Single-nucleotide mismatch selectivity of base-modified S-ONs

ON	Sequence	Base modifications	p27kip1 IC50, μM
			Wild-type	Mutant
8	UGG CUC UCC UGC GCC	pC, pU	0.20	0.25
4	TGG CTC TCX TGC GCC	G-clamp	0.05	0.25

Boldface indicates the position of the substituted heterocycle modification. S-ON modifications are shown in Fig. 1. The ON 8 contains C5-propynyl (pC, pU) in the bold positions and in 4, X = G-clamp. All linkages are phosphorothioate. T, thymidine; G, 2′-deoxyguanosine; C, 2′-deoxy-5′-methylcytidine. IC₅₀ values were determined by microinjection as described (15, 16, 17). Each experiment was repeated in triplicate. The wild-type and mutant p27^kip1 sequences differed by a single nucleotide. The guanine base at position 312 based on the human sequence (GenBank accession no. U10906) was changed to a thymidine. Sequences are: G-clamp S-ON, 3′-CCG CGT XCT CTC GGT; p27^kip1 wild-type RNA, 5′-GGC GCA GGA GAG CCA (human sequences 306–320); p27^kip1 mutant RNA, 5′-GGC GCA UGA GAG CCA (italicized nucleotide denotes the mismatch).