in the absence of its specific receptor,the virus cannot adsorb and hence cannot infect. Moreover,if the receptor is altered,for example, by mutation, the host may become resistant to virus infection. However,mutants of the virus can also arise that gain the ability adsorb to previously resistant hosts. In addition,some animal viruses may be able to use more than one receptor,so the loss of one may not necessarily prevent attachment. Thus,the host range of a particular virus is,to some extent,determined by the availability of a suitable receptor that the virus can recognize. In multicellular organisms,cells in different tissues or organs often express different proteins on their cell surfaces.