TCN2 and osteoporosis: Therefore, we chose four well-known SNPs of B vitamins-related genes, including TCN2 (encodes TC II), CD320 (encodes TC II receptor), SLC19A1 (encodes reduced folate carrier gene (RFC1)), and SLC19A2 (encodes thiamine carrier 1) because polymorphisms of B vitamins-related genes could reduce the availability of B vitamins contributing to the risk of osteoporosis and OVCFs [16,17].