SMAD9 and neural tube defect: Next, we detected BMP pathway-related genes in NTD samples, including BMP pathway-related receptors, ActRII, ALK2, ALK3, and ALK6; BMP ligands, BMP2 and BMP4; BMP pathway inhibitors, chordin and LRP2; BMP pathway target genes, GATA2, ID1, ID2, and noggin; BMP pathway downstream effector molecules, Smad1, Smad4, Smad5, Smad8; and other BMP pathway-related genes, such as Smurf1 and Smurf2. The results showed that expression of chordin was significantly increased and ID2 was significantly decreased (Fig. 7d) (p < 0.05).