A codon variant that changes at least one base of the first codon of a transcript.
Comment:
This is being used to annotate changes to the first codon of a transcript, when the first annotated codon is not to methionine. A variant is predicted to change the first amino acid of a translation irrespective of the fact that the underlying codon is an AUG. As such for transcripts with an incomplete CDS (sequence does not start with an AUG), it is still called.