For nsp1, the fact that the carboxy-terminal but not the amino-terminal half of the protein can be deleted suggests that there may be specific and distinct domains within the protein or that the entire protein is dispensable but that the RNA encoding the amino-terminal half of nsp1 cannot be deleted. The identification of specific required residues support the conclusion that it is the portion of the protein that is required for replication. The results of mutagenesis of the nsp14 coding region and flanking cleavage sites also provided important new insights into this protein and its requirements. Our previous study raised the question as to the essential nature of nsp14 in replication. The results of this study show that putative active site residues cannot be substituted without loss of replication in culture. Interestingly, mutagenesis of Tyr414 showed that while this residue can tolerate a number of substitutions, it was intolerant of Lysine or deletion. The results suggest that nsp14 is required for replication. However, whatever functions nsp14 serves appear to be retained by noncleaved or partially processed nsp14, since abolition of either the amino-terminal or carboxy-terminal cleavage site allowed recovery of viable virus.