求助：Fc受体CD16a里的变异体，176和158这两个数字分别表示什么意思

158我知道，是第158个氨基酸，那么176是代表什么呢

看到一个商品描述是这样的：

The Fc-Gamma Receptors (FCGRs) are members of immunoglobulin superfamily and play a critical role in the function of therapeutic antibodies. FCGRs are divided into three classes and FCGR3 (CD16) is expressed as two distinct forms (FCGR3A and FCGR3B) encoded by two different highly homologous genes in a cell type specific manner. FCGR3A is a low/intermediate affinity receptor for polyvalent immune-complexed IgG. It is involved in phagocytosis, secretion of enzymes and inflammatory mediators, antibody-dependent cellular cytotoxicity (ADCC), mast cell degranulation and clearance of immune complexes. In humans, a single nucleotide polymorphism creates two isoforms: high binding (176Val/V158) and low binding (176Phe/F158) forms that, when homozygous, may influence susceptibility to autoimmune diseases or response to therapeutic IgG antibodies. FCGR3A has been considered as an important therapeutic target. This AlphaLISA assay can be used to determine the binding activity of human IgG Fc fragment to human FCGR3A and also can be used to study how other antibodies bind to FCGR3A by competition assay.