to ty3 specific and sigma specific probes see fig4 fig4 a illustrates donor plasmids containing galactose inducible ty3 1 and ty3 2 sequences which were constructed as described above and are shown generalized as pegty3 yeast sequences marked are gal1 10 uas 20 solid sigma elements 22 lines and stippling ty3 internal domain 24 dark stippling 2 micron plasmid replication sequences 26 thin lines and ura3 sequences 28 thick lines e coli sequences ori and ampr are also shown 30 open spaces scale is approximate vectors may be constructed utilizing the position specific endonuclease of s cerevisiae using any of the well known vector construction techniques those techniques however are modified to the extent that the gene to be inserted into the genome of the host cell is flanked on either side by the marker sequences discussed herein and the position specific endonuclease disclosed herein that recognizes those markers must also be provided in the host cell the position specific endonuclease may be provided directly in the cell in its polypeptide form or alternatively it may be provided in the cell in the form of an expressible nucleotide sequence either rna or dna in accordance with techniques recognized in the art the vector itself may be of any suitable type such as a viral vector rna or dna naked straight chain or circular dna or a vesicle or envelope containing the nucleic acid material and any polypeptides that are to be inserted into the cell with respect to vesicles techniques for construction of lipid vesicles such as liposomes are well known such liposomes may be targeted to particular cells using other conventional techniques such as providing an antibody or other.