Lineage analysis by mapping of MDV-induced T-cell lymphomas indicates a predominately, but not exclusively, monoclonal origin. The mapped MDV integration sites of 15 tumors from 6 birds (4 males and 2 females) provide evidence for several different modes of clonality, predominant being monoclonal. Four of the birds (Bird 1, 2, 5, and 13) displayed tumors indicating a monoclonal origin, and one of the four (Bird 1) exhibited a derived cell population. Tumors from Bird 3 possessed both in-common integration sites and unique sites for each tumor, potentially indicating a polyclonal origin. Bird 12 also displayed two tumors with a polyclonal origin, and a derived cell population in 12-T2. Both tumors from Bird 12 also have three unidentified integration sites and they could possibly be in-common sites. Each tumor is represented by a colored shape, with the tumor number indicated (see Table 3), which corresponds to the symbols found in Figure 5. The same shape and color tumor indicates identical integration sites between those tumors. Tumors that contain both in-common and unique sites (Bird 3) are represented with different shapes and the same color. If a tumor exhibited a derived cell population (1-T2, 1-T3, and 12-T2) it is depicted as a quarter section of the circle filled with a different color. Those tumors with no in-common sites (Bird 12) are represented by a different color and shape.