Cyanine dye compounds having a substituted methine moiety that are nucleic acid stains, particularly for fluorescent staining of RNA, including compounds having the formula where R1 is a C1-C6 alkyl, sulfoalkyl, carboxyalkyl or C1-C6 alkoxy; each R2 is independently selected from the group consisting of H, C1-C6 alkyl, C1-C6 alkoxy, fused benzo, trifluoromethyl, amino, sulfo, carboxy and halogen, that is optionally further substituted; at least one of R3, R4, and R5 is an alkyl, aryl, heteroaryl, cyclic, or heterocyclic moiety that is optionally substituted by alkyl, amino, aminoalkyl, carboxy, nitro, or halogen; and the remaining R3, R4 or R5 are hydrogen; X is S, O, or Se; and D is a substituted or unsubstituted pyridinium, quinolinium or benzazolium moiety.