Thyroglobulin is synthesized by the follicular epithelial cells of the thyroid. Thyroglobulin is the glycoprotein precursor to the thyroid hormones. Its synthesis and metabolism have seemingly wasteful features. It has a molecular weight of 660,000, with 2 identical subunits of MW 300,000 and 10% sugars; yet its complete hydrolysis yields only 2 to 4 molecules of the iodothyroxines T4 and T3. Baas et al. (1986) showed that the TG gene encodes an 8.7-kb mRNA, covers at least 300 kb of genomic DNA and contains at least 37 exons separated by introns as large as 64 kb. A striking structural difference between the 5-prime and 3-prime parts of the gene suggested that it is composed of 2 evolutionarily different regions. The first 30 kb of DNA encodes 3 kb of the mRNA yielding an exon:intron ratio of 1:10, whereas the remaining 270 kb encodes 5.7 kb of the mRNA with a ratio of 1.47.