Napsin A is an aspartic proteinase that belongs to the peptidase A1 family and plays a role in pneumocyte surfactant processing. It is a 420-amino acid polypeptide consisting of a 24-residue signal peptide, a 40-amino acid propart, the mature enzyme of 336 amino acids, and a C-terminal extension of 18 residues. The mature Napsin A protein contains 3 predicted disulfide bonds, 3 potential N-linked oligosaccharide attachment sites, an RGD motif, a recognition motif for integrin binding, in the C terminus, immediately before a 4-amino acid insert that is unique to aspartic proteinases. Highest levels of Napsin A have been detected in adult lung (type II pneumocytes), fetal lung, and kidney tissues. Napsin A is also expressed at lower levels in adult spleen and at very low levels in peripheral blood leukocytes. Human napsin A shares 72.6% sequence identity with the mouse homolog.