The four Sendai virus C-proteins (C , C, Y1, and Y2) represent an N-terminal nested set of non-structural proteins whose expression modulates both the readout of the viral genome and the host cell response. In particular, they modulate the innate immune response by perturbing the signaling of type 1 interferons. The initiation codons for the four C-proteins have been mapped in vitro, and it has been proposed that the Y proteins are initiated by ribosomal shunting. A number of mutations were reported that significantly enhanced Y expression, and this was attributed to increased shunt-mediated initiation. However, we demonstrate that this arises due to enhanced proteolytic processing of C , an event that requires its very N terminus. Curiously, although Y expression in vitro is mediated almost exclusively by initiation, Y proteins in vivo can arise both by translation initiation and processing of the C protein. To our knowledge this is the first example of two apparently independent pathways leading to the expression of the same polypeptide chain. This dual pathway explains several features of Y expression.The Sendai virus P/C mRNA has become a paradigm for expressional elasticity, expressing six different polypeptide chains by ribosomal choice (namely, CЈ, P, C, Y1, Y2, and X) (1). The P protein initiates at the second start site (AUG 104 ). It is an essential cofactor of the viral polymerase (2). The first start site is an unusual ACG initiation codon at position 81 (ACG 81 ) that gives rise to the CЈ protein (3,4). This is a member of an N-terminally nested set of four proteins referred to as the "C-proteins" whose open reading frame (ORF) 1 overlaps that of P. The second member of this group, the C protein, initiates at AUG 114 and is accessed by leaky ribosomal scanning (5). Its good Kozak consensus signal is consistent with its high expression levels in virally infected cells. It is also the major translation product when the P/C mRNA is expressed transiently in mammalian cells or in rabbit reticulocyte lysates (RRLs). The remaining members of this group are the Y proteins (Y1 and Y2). Their AUG start codons were mapped in vitro to positions 183 and 201 (6). The C-proteins impact on both the readout of the viral genome (7,8) and the host cell response to viral infection. In particular, they function to disrupt type 1 interferon signaling pathways (9, 10).Although initiation from the first three start sites (ACG 81/CЈ , AUG 104/P , and AUG 114/C ) is readily explained by leaky scanning, results suggest that ribosomes can access the Y start codons by discontinuous scanning or shunting (11,12). The seminal observation in this work was that changing the CЈ ACG codon to AUG (referred to as AUG81) ablated expression of the P and C proteins but not Y1/Y2. In discontinuous scanning, ribosomes are loaded via the 5Ј cap and then at a defined donor site translocate to an internal acceptor site located close to the start codon (13). The first description of a eukaryotic shunt was the SeV X protein (14). This is initiate...