Insertional mutagenesis of MAV. The organisation of the chicken CCN3 gene is represented at the top. The CCN3 chicken protein encode a putative secreted 36 KDa protein. Following post-translational modifications, a full length 41 KDa CCN3 protein is released in the culture medium. In one MAV-induced nephroblastoma, proviral sequences were integrated within exon 2 of the CCN3 gene. As a consequence, MAV LTR was driving the expression of a chimaeric mRNA species encoding an amino truncated CCN3 protein deprived of module I and signal peptide. ORF : open reading frame ; (th) : theoretical ; (s) :secreted. The signal peptide (sp) is encoded by exon 1. E1–E5 : exons encoding the CCN3 protein. n201, n318 indicate the boundaries of the RNA stretch corresponding to the remaining sequences from exon 2. 91 n : 91 nucleotides of viral origin ; 117 n exon 2 sequences which are out of frame in the chimaeric mRNA species.