Lymphoid leukosis (LL) is a naturally occurring neoplastic disease of chickens, caused by avian lymphoid leukosis virus (ALV). Most inbred lines of chickens, susceptible to virus infection, are also susceptible to LL development. An interesting exception is line 63 which rarely develops LL after virus infection. To obtain more information concerning the resistance of line 63 to LL, we have compared virus infection and integration in bursal tissues of resistant line 63 and a susceptible cross (1515 × 71) after virus inoculation into day-old chicks. We report here that (a) the bursa tissues in both lines are infected to similar extent and (b) the initial viral infections in both lines lead to "random" integration of proviruses in the chromosomes of the bursal tissues. Only in the tumors developing later in the susceptible line can specific integration of provirus near a cellular oncogene, c-myc, be detected. Based on these data, the mechanism of resistance of line 63 to LL is discussed.
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