Two GFP molecules are capable of forming anti-parallel dimers. Two molecules of TetR-GFP can easily dimerize with each other, thus pairing two tetO arrays. Likewise, two molecules of GFP-LacI can easily dimerize with each other, thus pairing two lacO arrays. However, a TetR-GFP molecule cannot easily dimerize with a GFP-LacI molecule because of steric clash between TetR and LacI, which occurs because TetR and LacI will be on the same side of the dimer (as opposed to TetR-GFP/TetR-GFP or GFP-Lac/GFP-LacI dimers, in which two repressor molecules will be on different sides of the dimer).