In another aspect, the invention includes an oligonucleotide selected from OP(???O)(O--)O--(5')(B)sRRRR(B)wBt* and OP(???O)(O--)O--(5')(B)sRRRR(B)w(3')OP(???O)(O--)O, wherein B is a deoxyribonucleotide or an analog thereof; R is a ribonucleotide; s is in the range of from 1 to 8; w is in the range of from 0 to 8, such that the sum of s and w is less than or equal to 8; and Bt* is a labeled, non-ex