Construction of HN stalk domain mutant NDVs. Four amino acids (alanine, leucine, proline, and leucine at amino acid positions 89, 90, 93, and 94, respectively) in the HN stalk domain were individually mutated into glutamine or alanine. Among the four constructs, one rNDV with substitution of glutamine for alanine (rNDV/HN-A89Q) was recovered.