Frame: The frame of this exon. Contains a value of 0, 1, or 2, which indicates the offset of the first coding base of the exon within the reading frame of the coding DNA sequence, if any. This field is dependent on the strandedness of this annotation (see Annotation.reverse_strand). For forward stranded annotations, this offset is relative to the exon.start. For reverse strand annotations, this offset is relative to the exon.end `- 1`. Unset if this exon does not intersect the coding sequence. Upon creation of a transcript, the frame must be populated for all or none of the coding exons.