Inhibitor of serine proteases. Expressed predominantly in epididymis where it is found in the epithelial cells of the caput, corpus and cauda epididymis. Expression increases gradually from post-natal day 1 to week 2, increases significantly from week 2 to week 4 and remains high thereafter. Expression decreases following castration. Administration of testosterone restores expression levels. The reactive center loop (RCL) extends out from the body of the protein and directs binding to the target protease. The protease cleaves the serpin at the reactive site within the RCL, establishing a covalent linkage between the carboxyl group of the serpin reactive site and the serine hydroxyl of the protease. The resulting inactive serpin-protease complex is highly stable. Variability within the reactive center loop (RCL) sequences of Serpina1-related genes may determine target protease specificity (By similarity). Murine alpha-1-antitrypsin is represented by a cluster of up to 6 individual Serpina1-related genes. The precise complement of Serpina1-related genes present varies according to the strain of the animal. Belongs to the serpin family.