All mouse cell lines commonly used for antibody or recombinant protein production are derived from the MOPC21 tumor of a female BALB/c mouse. This indicates that all cell lines, clones, and subclones which are derived from the MOPC21 tumor (the plasmacytomas P3X63.Ag8.653, NS1, NS0, and the hybridoma SP2/0.Ag14) produce X-MLV (Froud et al., 1997) .