In vivo excision and amplification of pre-determined, large genomic segments, directly from the genome of a natural host could be a novel cloning method for gene therapy. In this study, an in vivo excision and amplification system for human cells was devised with the Cre/loxP system of bacteriophage P1 and the large T antigen/SV40 on system of Simian virus 40. Two loxP sequences, each of which serves as a recognition site for recombinase Cre, were integrated unidirectionally into the 51- and 3``-end regions of the CFTR gene. SV40 on sequence, which serves as a conditional replication system, was inserted between the loxP sequences. Trans-acting gene cre and large T antigen, each of which was under the control of a tetracycline responsive promoter, were also inserted into the 5``- and 3``-end regions of the CFTR gene, respectively, by homologous recombination. The human BJAB cell line carrying the inserted elements was stably maintained until the excision and amplification functions were triggered by the induction of cre and large T antigen. Upon induction by doxycycline, the 250-kb CFTR genomic region of human chromosome 7 flanked by two loxP sites was excised. Our method is very useful for obtaining large genomic fragments in quantities directly from a natural host without using foreign hosts.