Genetic signatures: chromosomal loci linked to T-T4 and F-T4 in RI mouse strains aged 5–13 weeks (CXB, BXH, BXD, and AXBXA, the latter abbreviated AXB). For simplification, approximate locations are provided for loci (precise locations are in Tables 1–3; Supplemental Table 2). Solid bars, Significant or suggestive linkage. Speckled bars, Significant linkage from combined analysis. *, Loci identified in UM-HET3 mice (27). Similar to loci identified in humans: ^, for TSH (26); #, for F-T4 (25); +, Dio1 (humans and mice). Note: In UM-HET3 mice, the Chr 4 locus (D4Mit155, 102.069572 Mb) (27) is part of the broader region (100–114 Mb) linked to deiodinase activity in BXH mice and, albeit weakly, to T-T4 variability in CXB and BXH strains. This D4Mit155 locus is closer to Pde4b (101.92 Mb), a cAMP-specific phosphodiesterase, rather than to Dio1 (106.96 Mb).