2'-deoxy-5-methylisocytidine is widely used in assays to personalize the care of patients infected with HIV, hepatitis C, and other infectious agents. However, oligonucleotides that incorporate 2'-deoxy-5-methylisocytidine are expensive, because of its intrinsic chemical instability. We report here a C-glycoside analog that is more stable and, in oligonucleotides, pairs with 2'-deoxyisoguanosine, contributing to duplex stability about as much as a standard 2'-deoxycytidine and 2'-deoxyguanosine pair.