To investigate whether these insertions also occur in non-hMPV sequences, a GFP gene was inserted into the sixth gene position between the M2 and SH transcription units of rhMPV and rhMPV/ΔM2-2. An assay was developed to detect insertions, by designing a GFP ORF with an 11-nt sequence, CGA 6 TTA, positioned downstream of the first two GFP codons. This resulted in a frame shift in the downstream reading frame and a premature translational stop codon at the 6 th GFP codon, abrogating expression of GFP ( Figure 1B ,C and 1D). The modified GFP ORF is labeled GFPpolyA ( Figure 1D ). Insertion of a single nucleotide (or 4, 7, 10, etc.) in the A 6 tract of the CGA 6 TTA sequence would restore the translationally silenced GFP ORF, resulting in a fluorescent hMPV infectious focus. Four full-length cDNA's were engineered to recover