(A) Sex difference in Esr1 and Ucp1 expression and (B) mtDNA copy number in BAT of male and female WT mice (n = 5 to 6 mice per sex). (C) Esr1 expression is induced in BAT of WT female mice during cold challenge (5 hours, 4°C) versus room temperature (RT). (D) Expression of Esr1, Polg1, and Polg2 in HFD-fed or with genetic obesity (LepOb/+) with NC-fed WT mice (n = 5 to 6 mice per group). (E) Confirmation of ERα deletion in BAT from female ERαKOBAT mice (n = 6 per genotype). (F) Ucp1 expression in BAT at room temperature ERαKOBAT and impaired Ucp1 induction during cold challenge (5 hours, 4°C) in female ERαKOBAT mice versus Controlf/f. (G) Body weight during early HFD feeding of female versus Controlf/f (n = 5 to 6 per genotype). (H) WAT, inguinal (iWAT) and gonadal (gWAT), in female ERαKOBAT versus Controlf/f under normal chow (NC) and high-fat diet (HFD) feeding (n = 5 to 6 mice per genotype). (I) Body temperature in ERαKOBAT versus Controlf/f over time during cold challenge (5 hours, 4°C). (J) Increased lipid droplets in BAT from ERαKOBAT versus Controlf/f detected by histochemistry (n = 3 per genotype). (K) Transmission electron microscopy showing mitochondrial architecture in ERαKOBAT versus Controlf/f, with mitochondrial images quantified for (L) perimeter and (M) area. (N) mtDNA copy number determined by quantitative PCR (qPCR) in BAT from male and female ERαKOBAT versus Controlf/f (n = 5 to 6 mice per genotype; normalized to 1.0). (O) Polg1 expression in BAT of female ERαKOBAT versus Controlf/f. (P to S) Immunoblots and corresponding densitometry showing (Q) parkin protein expression, (R) Drp1 total protein, and (S) Drp1Ser600 phosphorylation in ERαKOBAT versus Controlf/f (n = 5 to 6 mice per genotype). Data are means ± SEM. Student’s t test or one-way ANOVA, *P < 0.05 between the genotypes or sexes. #P < 0.05 within group and between conditions.