TK15, a mutant derived from a temperature-sensitive mutant of Rous sarcoma virus (tsNY68), has extremely low infectivity although it has intact viral genes. Biological and biochemical analyses of the virus and virus-induced transformants showed that the mutant has a defect in the packaging of its own genomic RNA possibly owing to a deletion near the 5' end (S. Kawai and T. Koyama, J. Virol. 51:147-153, 1984; T. Koyama, F. Harada, and S. Kawai, J. Virol. 51:154-162, 1984). Nucleotide sequence analysis of the provirus DNA of the mutant revealed that the deletion extends from the 3' end of the primer binding site to 22 bases upstream of the gag initiation codon and also suggested that possible binding between an extra region of the primer molecule and the viral genome resulting from the deletion causes another defect in the replication of the TK15 genome. It was suggested that the deletion of 237 bases in TK15 was generated during reverse transcription of the genome by the skipping of a sequence between identical 13-base sequences present in the primer binding site and 35 to 22 bases upstream of the gag initiation codon of the parental virus.