Plays an important role in cell protection against oxidative stress and in the regulation of redox-related calcium homeostasis. Regulates the calcium level of the ER by protecting the calcium pump ATP2A2 against the oxidoreductase ERO1A-mediated oxidative damage. Within the ER, ERO1A activity increases the concentration of H(2)O(2), which attacks the luminal thiols in ATP2A2 and thus leads to cysteinyl sulfenic acid formation (-SOH) and SEPN1 reduces the SOH back to free thiol (-SH), thus restoring ATP2A2 activity (PubMed:25452428). Acts as a modulator of ryanodine receptor (RyR) activity: protects RyR from oxidation due to increased oxidative stress, or directly controls the RyR redox state, regulating the RyR-mediated calcium mobilization required for normal muscle development and differentiation (PubMed:19557870, PubMed:18713863). Essential for muscle regeneration and satellite cell maintenance in skeletal muscle (PubMed:21131290). Interacts with RYR1, RYR2 and RYR3 (PubMed:18713863). Isoform 1 and isoform 2 are expressed in skeletal muscle, brain, lung and placenta. Isoform 2 is also expressed in heart, diaphragm and stomach. The N-terminus (first 61 amino acids) contains an endoplasmic reticulum addressing and retention targeting signal. N-glycosylated. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The UGA codons present in position 127 and 462 are either a selenocysteine or a real stop codon. The UGA codon present in position 428 is either a selenocysteine or a real stop codon. Truncated N-terminus. Truncated N-terminus.