Amino acid sequence alignment of the C-termini of Craniata QSOX1 and QSOX2. Representative QSOX1 and QSOX2 sequences are shown. The positions of helices (α4, α5) from the Erv domain are indicated as black rectangles. The blue diagonal-hashed rectangle indicates a segment predicted to be helical. A stretch of basic amino acids downstream of the predicted helix is speculated to be a protease cleavage site for QSOX1 processing based on mass spectrometry fingerprinting of secreted QSOX1 (Ilani T, Alon A, Grossman I, Horowitz B, Kartvelishvily E, Cohen SR, Fass D: A secreted disulfide catalyst controls extracellular matrix composition and function, submitted) and the preponderance of basic amino acids in recognition sites for secretory pathway proteases such proprotein convertases.