Sequences of the attB, attP, attL and attR sites used by SV1 integrase. Panel A. The crossover site occurs within the 4 bp sequence in black and the positions of the inverted repeats are underlined in the attP and attB sites. Panel B. The results of a BLAST search for the SV1 attB site in Streptomyces genomes deposited in the nucleotide sequence database. The default settings did not identify an SV1 attB site from S. venezuelae or S. albus.