Complete genomes of the two most studied APVs, FWPV US (FP-challenge virus; Animal Health Inspection Service Centre for Veterinary Biologicals, Ames Iowa, USA), FWPV-FP9 (a plaque-purified tissue cultureadapted attenuated European virus) and a CNPV virulent strain (Wheatley C93, American Type Culture Collection; ATCC VR-111) have been sequenced [7, 8, 27] . Although nucleotide and amino acids sequences for these two viruses are known, the functions of some putative genes and proteins remain to be fully assigned. Comparison of the FP9 strain with FWPV US revealed 118 differences; of which 71 genes were affected by deletion (26 of 1-9334 bp), insertion (15 of 1-108 bp), substitution, termination or frame-shift [27] . FP9 strain is a derivative of European FWPV HP1 which was obtained through over 400 passages in chicken embryo fibroblast (CEF). Analysis of FWPV HP1 sequences at the loci in which differences exist between FP9 and FWPV US show that 68 of 118 loci differ from the FWPV US, but were identical to FP9. This thus indicate that more than half of the differences between the two geographic FWPV lineages represented differences between the parent virulent viruses FWPV HP1 and FWPV US [27] . Further comparison of molecular data; show that FWPV and CNPV share high aminoacid identity, significant gene-sequence rearrangements, deletions and insertions [8] . The CNPV genome is about 80-100 kbp larger than the FWPV genomes. Both FWPV and CNPV express cellular gene homologues with immunomodulatory functions, which might be responsible for their different virulence and host-range [8] , but CNPV shows a broader tissue tropism in permissive avian hosts [17] than FWPV. CNPV has additional sequence of over 75 kbp, 39 genes lacking in FWPV homologues and approximately 47% amino-acid divergence [8] . These divergences are primarily found in the terminal nonconserved regions [7, 8] . Genes located in the non conserved regions are more prone to mutation and recombination and are implicated in host range, immunomodulation and pathogenesis [28] , and may be responsible in some aspects of cell and/or tissue tropism or perform other cellular functions [8] .