Exon 1 was deleted after germline recombination of loxP sites. Western blot analysis confirmed the absence of full-length 46-kDa APMAP protein in all tested tissues of homozygous mutant mice. A second protein band of about 42 kDa was detected in all adipose tissue (AT) depots (epididymal WAT, stromal WAT, mesenteric and perirenal WATs, and brown AT) and was not affected by the deletion of exon 1. Mass spectrometry confirmed that the 42-kDa band detected by immunoblot analysis is a truncated version of APMAP protein. The mouse AT-specific APMAP isoform is independently transcribed from exon 2, as verified by 5' RACE analysis. The knock-out mouse model described in J:247647 lacks the full-length APMAP variant, i.e. the only isoform expressed in humans.