BMC Genomics. 2010 Jul 2;11:413. doi:10.1186/1471-2164-11-413

Kapteyn J, He R, McDowell ET, Gang DR.

Incorporation of non-natural nucleotides into template-switching oligonucleotides reduces background and improves cDNA synthesis from very small RNA samples.

TSO starting with iCiGiC (iso-dC / iso-dG) does not form concatenates because the RTase is inhibited by these nucleotides, therefore it does not reach the end and no extra template switching occurs.