All N TAIL constructs were obtained by PCR using the MV N gene, strain Edmonston B, as template. The N TAILHN gene construct, encoding residues 401-525 of the MV N protein with a hexahistidine tag fused to its N terminus, was obtained using the plasmid pET21a/N (encoding the MV N protein; Karlin et al. 2002a ) as template. Forward primer (5 0 -gatagaac catgCATCATCATCATCATCATactactgaggacaagatcagtaga-3 0 ) was designed to introduce a hexahistidine tag encoding sequence (upper case) at the N terminus of N TAIL , while reverse primer (5 0 -ggggaccactttgtacaagaaagctgggtcttagtctagaa gatttctgtcattgta-3 0 ) was designed to introduce an AttB2 site (bold). The PCR amplification product was further used as template in a second PCR step, using forward primer (5 0 -ggggacaagtttgtacaaaaaagcaggcttcgaaggagatagaaccatgCAT CATCATCAT-3 0 ), designed to introduce an AttB1 site (bold), and reverse primer as above. After purification (PCR Purification Kit; Qiagen), the PCR product was cloned into the pDest14 vector (Invitrogen) using the Gateway recombination system (Invitrogen). The final construct is referred to as pDest14/N TAILHN .