Nucleotide sequence analysis of two simian virus 40 early mutants dl1263 and dl1265, which lack a DNA segment around map positions 0.21 and 0.18, respectively (C. Cole, T. Landers, S. Goff, S. MAnteuil-Brutlag, and P. Berg, J. Virol. 24:277--294, 1977), revealed in-phase deletions of 33 nucleotide