Nucleotide sequence analysis of two si 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 pairs for dl1263 and 39 nucleotide pairs for d11265. The 33-base-pair deletion in dl1263 does not correspond to an apparent shortening by 6,000 daltons observed for the mutant T antigen. In dl1265 the

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... al termination signal as well as most of the prolinerich terminal tryptic peptide has been removed, and the carboxyl terminus of the mutant T antigen is a series of three cysteine residues.