A 3kb Hpal-Spel fragment of the FPV genome was inserted into a EcoR1-HindIII site of a pUC18 vector. The homologous recombination site may be interrupting a possible open reading frame but is thought to be non-essential for viral replication and has no known function in the FPV.

A 175bp EcoRV fragment of the FPV genome was removed from this construct and replaced with the MG 40K and mcg3 coding sequences.

Regulatory sequences were also inserted into the vector. The MG 40K and mcg3 genes were each coupled with a synthetic Ps promoter, which emulates the consensus early/late promoter of poxvirus. A termination sequence derived from the gB gene of the Marek Disease Virus was also inserted into the pUC18 vector.

Furthermore, a MDV gB transit signal sequence was added to the 3' terminus of the of MG 40K and mcg3 coding sequences to ensure localisation  to the cell membrane.

This construct and the attenuated parental FPV strain were co-transformed into chicken embryo fibroblasts (CEF) and incubated under conditions that favour homologous recombination.

After transfection, viral particles were cultivated from the chicken embryo fibroblasts and assayed for expression of MG 40K and mcg3 proteins. Plaques expressing MG 40K and mcg3 proteins were isolated and selected until the pure recombined virus was obtained.