 One of these sites initiates in the P open reading frame, whereas four others initiate a nested set of C proteins (C', C, Y1, Y2).These C proteins are initiated in the + 1 reading frame to that of P at different translation starting sites.Sendai virus uses ribosome shunting to express Y1 and Y2 proteins that initiate at the fourth and fifth start sites on the P/C mRNA (respectively).