Phenotypic differences between the two genetic backgrounds. H and E staining of (A) control, (B) homozygous mutant BALB/c, and (C) homozygous mutant C57BL/6. Note that in the C57BL/6 background, the phenotype is more severe with dysmorphic and involuted follicles (panel C white arrow), while in the BALB/c background, the follicles appear only enlarged (B) compared with the control. (D) Two-year-old mice control, (E) heterozygous BALB/c, and (F) heterozygous C57BL/6. The C57BL/6 heterozygous mice develop tumors at the thyroid level, while the BALB/c heterozygous mice present only with hypertrophic thyroid. (G) T4 and TSH hormone levels at PN14. In the C57BL/6 background, we observe a statistically significant decrease in T4 in both mutants and heterozygous animal, but a statistically significant increased level of TSH compared with the control was observed only in the mutant. In the BALB/c background, on the other hand, homozygous and heterozygous mutant levels are similar to the controls for TSH (n=5 for each group). Th, thyroid; Tr, trachea.