Another source of contamination is from cancerous cells infected by viruses (e.g., human papilloma virus). In such instances, some sequences that are assigned as 'human' are incorrectly annotated. In these instances that reflect the incorporation of the virus to the host, different protein sequences from the same virus are identified which are best explained as a result of infection or an integration event. For example, the proteins in UniRef90 clusters P06426, P06463, P21735, P06788, P36741 and P21736 belong to Human papilloma virus (Table 1) .