CLUSTAL X alignment of the ectodomains of NEP and Kell. The amino acid sequences and the numbering are from GenBank: Kell accession number M64934 and NEP J03779. The ectodomain sequence of NEP was from the pdb structure, 1DMT. Signature sequences, present in mammalian M13 zinc endopeptidases, and the Ww motif, present in most M13 peptidases listed in the MEROPS database³⁹  but absent in Kell, are boxed. The 10 conserved cysteine residues present in the M13 family are shaded. The 8 cysteines located in the domain closest to the membrane, which contains the active site, are marked by dots.