背景资料：CHRDL2 (chordin-related protein 2), also known as BNF1 (breast tumor novel factor 1) or CHL2 (chordin-like 2), is a 429 amino acid protein that belongs to the chordin family of proteins. CHRDL2 contains three VWFC (von Willebrand Factor type C) domains and is predominantly expressed in uterus and moderately expressed in prostate, liver, ovary, heart and testis. Due to alternative splicing events, CHRDL2 exists as five isoforms, namely isoform I, isoform II, isoform VII, isoform VIII and isoform IX. Two of these isoforms are secreted, while the other three localize to the cytoplasm. Functioning as a BMP-binding inhibitor, CHRDL2 directly interacts with BMPs and blocks their binding to BMP receptors, thereby inhibiting BMP activity. In breast, lung and colon tumors, CHRDL2 expression is upregulated, suggesting a possible role in tumorigenesis.