CST6 and melanoma: Some interesting relationships were observed between the different expression profiles of cystatin E/M, cystatin C, legumain, cathepsin B and L. First, in most melanoma cell lines examined, we identified an inverse correlation between secreted cystatin E/M and C. The only exception was the ocular melanoma cell line 92.1, in which neither of the cystatin isoforms was detected.