You can calculate its ATGC/CGTA distribution, email me, and I'll give you the method and the program, and three years ago, I entered a biometric contest, and the competition was to identify different bacteria
Transcript isoforms can contain isoform specific splice junctions - in some instances, individual exons and introns (yes, you can have retained introns in the mRNA) in the transcript may be shorter/longer than their canonical representations. And the forgoing isn't even considering isoform differences in 5' and 3' UTRs, the possibility of alternative TSSs and transcriptional RNA-editing in both pre-mRNA and mRNA.