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TSL (Transcript Support Level)

The Transcript Support Level (TSL) is a method to highlight the well-supported and poorly-supported transcript models for users. The method relies on the primary data that can support full-length transcript structure: mRNA and EST alignments supplied by UCSC and Ensembl.

It is important that users understand how to assess transcript annotations that they see in GENCODE. While some transcript models have a high level of support through the full length of their exon structure, there are also transcripts that are poorly supported and that should be considered speculative. 


The mRNA and EST alignments are compared to the GENCODE transcripts and the transcripts are scored according to how well the alignment matches over its full length. The GENCODE TSL provides a consistent method of evaluating the level of support that a GENCODE transcript annotation is actually expressed in humans. Human transcript sequences from the International Nucleotide Sequence Database Collaboration (GenBank, ENA, and DDBJ) are used as the evidence for this analysis. Exonerate RNA alignments from Ensembl, BLAT RNA and EST alignments from the UCSC Genome Browser Database are used in the analysis. Erroneous transcripts and libraries identified in lists maintained by the Ensembl, UCSC, HAVANA and RefSeq groups are flagged as suspect. GENCODE annotations for protein-coding and non-protein-coding transcripts are compared with the evidence alignments.

Annotations in the MHC region and other immunological genes are not evaluated, as automatic alignments tend to be very problematic. Methods for evaluating single-exon genes are still being developed and they are not included in the current analysis.

Multi-exon GENCODE annotations are evaluated using the criteria that all introns are supported by an evidence alignment and the evidence alignment does not indicate that there are unannotated exons. Small insertions and deletions in evidence alignments are assumed to be due to polymorphisms and not considered as differing from the annotations. All intron boundaries must match exactly. The transcript start and end locations are allowed to differ.


The following categories are assigned to each of the evaluated annotations:

  • tsl1 – all splice junctions of the transcript are supported by at least one non-suspect mRNA
  • tsl2 – the best supporting mRNA is flagged as suspect or the support is from multiple ESTs
  • tsl3 – the only support is from a single EST
  • tsl4 – the best supporting EST is flagged as suspect
  • tsl5 – no single transcript supports the model structure
  • tslNA – the transcript was not analysed for one of the following reasons:
    • pseudogene annotation, including transcribed pseudogenes
    • human leukocyte antigen (HLA) transcript
    • immunoglobin gene transcript
    • T-cell receptor transcript
    • single-exon transcript (will be included in a future version)