Abstract
Peptide antiserum and monoclonal antibodies have detected a previously unrecognized small non-structural protein in Sendai virus-infected cells. This protein, designated X, appears to represent the C-terminal 95 amino acids of the P protein reading frame. The X protein appears to be almost as abundant as the P protein on a molar basis in vivo. No evidence of a precursor-product relationship between the X and P proteins could be found.