BAT content and IR expression. Interscapular brown fat was obtained from WT, Cre, IRloxP, and BATIRKO mice at 3, 6, and 12 months of age. (a) BAT weight vs. body weight is represented. Results are expressed as a mean ± SEM (n = 10–20). ^#P < 0.00001, BATIRKO vs. IRloxP; **P < 0.0001, BATIRKO vs. IRloxP or Cre; *P < 0.01, BATIRKO vs. IRloxP or Cre or WT. Representative interscapular BAT from IRloxP and BATIRKO mice at 6 months of age (b) (upper panel). Hematoxylin and eosin stain of BAT from IRloxP and BATIRKO mice at 6 months of age (b) (lower panel). RT-PCR analysis of RNA prepared from BAT from BATIRKO mice at different ages (1 day and 1, 3, 6, and 12 months) and IRloxP at 6 months of age, to study IR expression. The RT reaction was carried out as described in Methods. A larger band (585 bp) was observed in the IRloxP lane, while a smaller band of 435 bp was observed in BATIRKO mice lanes, suggesting a recombination event (c) (upper panel). Protein extracts of BAT from IRloxP and BATIRKO mice at different ages (1, 3, and 6 months) were subjected to immunoprecipitation with an mAb against IR β-chain and analyzed by Western blot. This is representative of three experiments (c) (lower panel). Protein extracts from different tissues of 6-month-old WT, IRloxP, Cre, and BATIRKO mice were immunoprecipitated with the same Ab and analyzed by Western blot. This is representative of three experiments (d). φx, DNA marker.