All vertebrate ferritin genes have three introns and four exons. Alternative splicing of exon 5 gives rise to two alternate exons. The CCL2 gene has three exons and two introns. The PKM gene consists of 12 exons and 11 introns. CD44 splice variants containing variable exons are designated CD44v. Some elements of the genome with unique sequences function as exons, introns and regulatory DNA. The majority of eukaryotic pre-mRNAs consist of alternating segments called exons and introns. The M-gene consists of 12 exons and 11 introns. The CFTR gene is approximately 189 kb in length, with 27 exons and 26 introns. Nuclear pre-mRNA introns are often much longer than their surrounding exons. It spans 28 kpb, has 11 introns and 12 exons. Similarly, a gene's introns can be much larger than its exons. The human FOXP3 genes contain 11 coding exons. Multiple transcript variants, resulting from alternative splicing at two coding exons, have been well characterized. Expression can be quantified for exons or genes using contigs or reference transcript annotations.