CTSD and breast cancer: CTSD was initially linked to cancer in breast cancer,[15] and overexpression of CTSD was shown to alter its processing, cell localization, and secretion.[16] CTSD overexpression is now recognized as a marker of poor prognosis associated with clinical metastasis.[17] The distinct N‐glycosylation structure of CTSD in tumor tissues and serum in patients with breast cancer relative to normal tissues[18] suggests that varying N‐glycosylation patterns of CTSD may be closely linked to tumorigenesis.