Mutation can also occur in cells other than germ cells - somatic mutation ? but this is likely to be apparent only when the genetically altered cell proliferates abnormally to form a large family or clone of similar cells , e . g . when a tumour forms 突变也可发生于生殖细胞以外的细胞（体细胞突变） ，但这很可能仅表现于遗传性改变的细胞异常增生而形成一大系或克隆的相似的细胞时，例如当一个肿瘤形成时。
