Cre and FLP recognition sequences and mechanism of recombination. (A) The Cre recombinase recognition sequence is termed loxP (black arrow), while the Flp recombinase recognition sequence is termed FRT (gray arrow). Both loxP and FRT contain two 13-bp inverted repeats that are separated by an 8-bp spacer region. (B) Cre mediates excision or circularization of a DNA segment (white rectangle) when flanked by two loxP sites (black triangles) in a cis (same DNA strand) arrangement and oriented in the same direction. (C) Cre mediates inversion of a DNA segment (black arrow) when flanked by two loxP sites (black triangles) in a cis arrangement and oriented in the opposite direction. (D) Cre mediates translocation of a DNA segment (rectangles) when the loxP sites (black triangles) are located on different strands of DNA (trans arrangement) and are oriented in the same direction.