Genomic islands, flanked by attachment sites, devoid of conjugation and recombination modules and related to the integrative and conjugative element (ICE) ICESt3, were previously found in Streptococcus thermophilus. Here, we show that ICESt3 transfers to a recipient harbouring a similar engineered genomic island, CIMEL3catR3, and integrates by site-specific recombination into its attachment sites, leading to their accretion. The resulting composite island can excise, showing that ICESt3 mobilizes CIMEL3catR3 in cis. ICESt3, CIMEL3catR3 and the whole composite element can transfer from the strain harbouring the composite structure. The ICESt3 transfer to a recipient bearing CIMEL3catR3 can also lead to retromobilization, i.e. its capture by the donor. This is the first demonstration of specific conjugative mobilization of a genomic island in cis and the first report of ICE-mediated retromobilization. CIMEL3catR3 would be the prototype of a novel class of non-autonomous mobile elements (CIMEs: CIs mobilizable elements), which hijack the recombination and conjugation machinery of related ICEs to excise, transfer and integrate. Few genome analyses have shown that CIMEs could be widespread and have revealed internal repeats that could result from accretions in numerous genomic islands, suggesting that accretion and cis mobilization have a key role in evolution of genomic islands.