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Happel et al. J Cancer Metastasis Treat 2020;6:32 I http://dx.doi.org/10.20517/2394-4722.2020.71 Page 3 of 18

Figure 1. A schematic diagram showing exRNA types predominantly found in a representative set of human biofluids. miRNA: microRNA;
piRNA: piwi-interacting RNA; tRF: tRNA-derived RNA fragments

Table 1. General classification of non-coding RNAs
Group Abbreviation Full Name Size ncRNA Review Article(s)
Housekeeping ncRNAs
rRNA ribosomal RNA 120-4,500 nt [72,73]
snRNA small nuclear RNA 100-300 nt [74]
snoRNA* small nucleolar RNA* 60-300 nt* [75,76]*
tRNA transfer RNA 76-90 nt [77]
Regulatory ncRNAs
lncRNA long non-coding RNA > 200 nt [78]
miRNA* microRNA* 21-22 nt* [79,80]*
piRNA* piwi-interacting RNA* 23-31 nt* [81]*
siRNA small interfering RNA 20-25 nt [82]
tRF* tRNA-derived fragments* 17-26 nt* [83]*
Y RNA* Y RNA* ?* [84]*
*Asterisks represent the most prominent types of exRNAs found in human biofluids