instability and sitespecific excision of integrationproficient mycobacteriophage l5 plasmids development of stably maintained integrative vectors integrative vectors expressing foreign genes are used as tools for the development of recombinant vaccines in mycobacteria since it is assumed that these vectors are stably maintained even without antibiotic selection we here demonstrate that integrationproficient vectors are lost from the mycobacterial genome in high frequency loss of integrated vectors occurred in reca and in recastrains indicating a recaindependent mechanism loss of the integrated vector was prevented when integrase gene function was carried on a separate plasmid that is unable to replicate in mycobacteria indicating that excision is a function of integrase by providing attp in cis and integrase function in trans vectors integrating at the attb site are stably maintained even when carrying genes that deleteriously affect the host