The structure Siaα2,3(GalNAcβ1,4)Gal- is the epitope of the Sd^a antigen, which is expressed on the erythrocytes and secretions of the vast majority of Caucasians, carried by N- and O-linked chains of glycoproteins, as well as by glycolipids. Sd^a is very similar, but not identical, to ganglioside GM2 [Siaα2,3(GalNAcβ1,4)Galβ1,4Glc-Cer]. The Sd^a synthase β1,4 N-acetylgalactosaminyl transferase 2 (B4GALNT2) exists in a short and a long form, diverging in the aminoterminal domain. The latter has a very long cytoplasmic tail and displays a Golgi- as well as a post-Golgi localization. The biosynthesis of Sd^a is mutually exclusive with that of the cancer-associated sialyl Lewis antigens, whose structure is Siaα2,3Galβ1,3/4(Fucα1,4/3)GlcNAc-. B4GALNT2 is down-regulated in colon cancer but patients with higher expression survive longer. In experimental systems, B4GALNT2 inhibits colon cancer progression,not only through inhibition of sialyl Lewis antigen biosynthesis. By contrast, in breast cancer B4GALNT2 is associated with malignancy. In colon cancer, the B4GALNT2 gene is regulated by multiple mechanisms, which include miRNA and transcription factor expression, as well as CpG methylation. In addition, Sd^a/B4GALNT2 regulates the susceptibility to infectious agents, the protection from muscle dystrophy, the activity of immune system in pregnancy and the immune rejection in xenotransplantation.

结构 Siaα2,3(GalNAcβ1,4)Gal- 是 Sd ^a抗原的表位，它在绝大多数高加索人的红细胞和分泌物上表达，由糖蛋白的 N 和 O 连接链携带，以及如糖脂。Sd ^a与神经节苷脂 GM2 [Siaα2,3(GalNAcβ1,4)Galβ1,4Glc-Cer] 非常相似，但不完全相同。Sd ^a合酶 β1,4 N-乙酰半乳糖氨基转移酶 2 (B4GALNT2) 以短形式和长形式存在，在氨基末端结构域中不同。后者有一个很长的细胞质尾部并显示高尔基体和后高尔基体定位。^{Sd a}的生物合成与癌症相关的唾液酸 Lewis 抗原互斥，其结构为 Siaα2,3Galβ1,3/4(Fucα1,4/3)GlcNAc-。B4GALNT2 在结肠癌中下调，但表达较高的患者存活时间更长。在实验系统中，B4GALNT2 不仅通过抑制唾液酸 Lewis 抗原生物合成来抑制结肠癌的进展。相比之下，在乳腺癌中，B4GALNT2 与恶性肿瘤相关。在结肠癌中，B4GALNT2基因受多种机制调控，包括 miRNA 和转录因子表达，以及 CpG 甲基化。此外，^Sd/B4GALNT2 调节对感染因子的易感性、对肌肉营养不良的保护、妊娠期免疫系统的活性和异种移植中的免疫排斥。