As with many biological terms, the use of protein family is somewhat context dependent; it may indicate large groups of proteins with the lowest possible level of detectable sequence similarity, or very narrow groups of proteins with almost identical sequence, function, and three-dimensional structure, or any kind of group in-between. To distinguish between these situations, the term protein superfamily is often used for distantly related proteins whose relatedness is not detectable by sequence similarity, but only from shared structural features.[2][3][4] Other terms such as protein class, group, clan and sub-family have been coined over the years, but all suffer similar ambiguities of usage. A common usage is that superfamilies (structural homology) contain families (sequence homology) which contain sub-families.