*Maternally Expressed 3 (MEG3) lncRNA:*

pCI-MEG3 was a gift from Anne Klibanski (Addgene plasmid #44727, Watertown, MA, USA) [43]. Primers to obtain the DNA template for in vitro transcription were as follows:

T7 RNA promoter region followed by: MEG3 forward primer: TAATACGACTCACTATAGGGGCAGAGAGGGAGCGCGCCTTGG MEG3 reverse primer: GATATCTTTTTGTTAAGACAGGAAACACATTTATTGAGAGC

*50 nt RNA:*

50 nucleotide RNA was an artificial randomized RNA sequence. DNA templates were T7 promoter region followed by:

50 nt forward oligo:

```
TAATACGACTCACTATAGAAGAATGGCCTCGCGGAGGCATGCGTCATGCTAGCGTGCGGG
GTACTCTT and
```
50 nt reverse oligo:

AAGAGTACCCCGCACGCTAGCATGACGCATGCCTCCGCGAGGCCATTCTTCTATAGTGAG TCGTATTA

Transcribed RNA:

GAAGAAUGGCCUCGCGGAGGCAUGCGUCAUGCUAGCGUGCGGGGUACUCUU

All primers and oligonucleotides were purchased from Sigma-Aldrich Ltd. (St. Louis, MO, USA). Tested RNAs were synthesized by in vitro transcription carried out with New England BioLabs HiScribe™ T7 Quick High Yield RNA Synthesis Kit (Ipswich, MA, USA). Fluorescein-labelled, single-stranded RNA probes were generated by using Roche (Basel, Switzerland) Fluorescein RNA Labeling Mix (11685619910) and NEB 10× T7 reaction buffer (#B2041A). After transcription, remaining DNA templates were eliminated with DNaseI treatment. RNA sample purification was carried out using Macherey-Nagel NucleoSpin® RNA Clean-up XS Kit (Düren, Germany). The quality and intactness of the purified transcription products were analysed by native and formaldehyde agarose gel electrophoresis.

Biotinylation of the RNAs was performed using Pierce™ RNA 3 End Biotinylation Kit (Cat. Number 20160, Thermo Fisher Scientific, Waltham, UK) according to the instructions of the manufacturer. Overnight incubation at 16 ◦C was applied for the ligation of the biotin label. Final RNA concentrations were determined using NanoDrop™ 1000 Spectrophotometer (Thermo Fisher Scientific, Waltham, UK).

Purified RNAs were stored −80 ◦C until usage in the presence of RNAINH-RO Roche Protector RNase Inhibitor (20U).
