hz5 at njit.edu (haibo) wrote > For all I know, besides the homolog part, the key difference is that ortholog is > same function in different species, while paralog is different function in same > species. That is a probable consequence of orthology and paralogy, not the definition. With ancient paralogs (such as hemoglobin and myoglobin), it is a useful property. For recent paralogs (such as olfactory receptors in rodents) it may not be that useful. The definitions I've seen are all a little bit vague---they do not handle well large gene families in which a lot of duplication and gene elimination occur. In many cases, even if the definition is clear, we often have no way of determining whether or not two members of such a family in different species are orthologs. The closest matches between the species may have arisen from different copies in the ancestral species, and hence be paralogs rather than orthologs. Kevin Karplus karplus at soe.ucsc.edu http://www.soe.ucsc.edu/~karplus Professor of Computer Engineering, University of California, Santa Cruz Undergraduate and Graduate Director, Bioinformatics Affiliations for identification only.