at position 5670 2 in the lav dna sequence the invention further concerns dna sequences which provide open reading frames defined as orf q orf r and as u201c 1 u201d u201c 2 u201d u201c 3 u201d u201c 4 u201d u201c 5 u201d the relative position of which appears more particularly in fig2 and 3 the ltr long terminal repeats can be defined as lying between position 8560 and position 160 end extending over position 9097 1 as a matter of fact the end of the genome is at 9097 and because of the ltr structure of the retrovirus links up with the beginning of the sequence the invention concerns more particularly all the dna fragments which have been more specifically referred to hereabove and which correspond to open reading frames it will be understood that the man skilled in the art will be able to obtain them all for instance by cleaving an entire dna corresponding to the complete genome of a lav species such as by cleavage by a partial or complete digestion thereof with a suitable restriction enzyme and by the subsequent recovery of the relevant fragments the different dnas disclosed in the earlier mentioned british application.