We have previously reported that a virus, MH2-PA200, lacking the ability to transform quail embryo cells, could be isolated from wild type (wt) MH2 stocks passaged on chicken neuroretina cells. We report here the molecular cloning and extensive characterization of this MH2-PA200 provirus. Molecularly cloned MH2-PA200 DNA was found to stimulate the growth of neuroretina cells by transfection assays and our results indicate that this recombinant virus was derived from the RAV-1 helper virus, in which v-mil and a small part of v-myc of MH2 were acquired at the expense of helper (delta gag-pol-delta env) sequences. In order to assess the precise boundary between the myc and env genes we determined the nucleotide sequence of the junction fragment and showed that 11 of 13 nucleotides of the env gene were identical to the myc sequence at the recombination point. The nucleotide sequence of the myc-env junction fragment of another similar and independently generated MH2 mutant showed similarly 9 nucleotides of homology between the env and myc sequences at the recombination point that took place at another site, suggesting that a homologous recombination occurred between MH2 and RAV-1 viruses to generate MH2-PA200 and similar mutants.