First, transcripts from different sources are added to the database using custom import scripts. Two transcripts of which all the exon positions are identical, as such creating a nonredundant transcript collection. After each import, the chromosomal positions of the newly added exons are converted to either hg19 or hg38 using the liftOver tool and corresponding chain files provided on the UCSC genome browser website (https://genome.ucsc.edu/cgi bin/hgLiftOver). Only conversions with perfect remapping of all bases are considered. Next, the metadata (conservation, coding potential, HGNC gene symbols, …) for each transcript is either obtained or generated. In the filtering step, the following transcript are removed from the database: