SBC oligos exhibit high affinity for natural oligonucleotides but they show little affinity for other SBC oligos even of a complementary sequence. Oligos in which A has been replaced with 2-amino-A and T with 2-thio-T represent an excellent example of SBC oligos. While 2-amino-A forms a very stable base pair with T containing three hydrogen bonds, the stability of the base pair with 2-thio-T is greatly diminished. However, 2-thio-T base pairs perfectly well with A. As an example, SBC 20mers annealed against a DNA 20mer target exhibited Tm values 10 °C higher than the corresponding DNA-DNA hybrid, whereas the SBC-SBC hybrid yielded Tm values 30 °C lower.