Contrary to mouse, may not be involved in the beneficial effects of muscular exercise, nor in the induction of browning of human white adipose tissue. Imported in peroxisomes through the PEX5 receptor pathway. Detected in the blood of individuals subjected to endurance exercise. Widely expressed, with highest levels in heart. Very low expression, if any, in colon, pancreas and spleen. Has been shown to be up-regulated some twofold by muscular exercise at the mRNA and protein level; this effect has been suggested to be mediated by PPARGC1A (PubMed:22237023). However, up-regulation upon exercise could not be reproduced, at least not at the mRNA level (PubMed:24040023). The extracellular domain is cleaved and released from the cell membrane. N-Glycosylated. It is uncertain whether Met-1 or Met-76 is the initiator. Transcript evidence available at present time points at Met-76. In this short version of the protein, the signal peptide cannot be predicted and the irisin peptide is severely truncated. The first initiation codon corresponds to a non-AUG site, an N-terminal ATA codon. This initiation codon has been annotated as it is in good Kozak context and it is conserved in other primates, including gibbon, chimpanzee and gorilla. Although the existence of such a form has not been demonstrated in human, western blot analysis following endurance exercise shows the presence of an irisin peptide of similar size in human and mouse plasma (PubMed:22237023), suggesting the existence of full-length irisin in human, at least in some tissues. However, expression from Met-1 may be less efficient than that from Met-76 (PubMed:24040023).