NP1 mutants exhibit a greater defect in viral replication than a capsid-lacking mutant. Southern blot analysis of samples taken from WRD cells infected with MVC (lane 1) or transfected with one of the following: WT pIMVC (lane 2), the NP1 mutant (NP1-2735TAG) (lane 3), NP1-5XPro (lane 4), or a capsid-lacking mutant described in Materials and Methods (lane 5). The infection or transfections were done in the presence of a neutralizing antibody, and Southern blot analyses were done as described in the text. Total DNA content was measured, and equivalent amounts of total DNA were loaded in each lane. DNA replication forms are indicated by monomer, dimer, and single-stranded (SS) DNA.