Annealing of the 3 end of one primer to itself or the second primer may cause primer extension, resulting in the formation of so-called primer dimers, visible as low-molecular-weight bands on PCR gels. Primer dimer formation often competes with formation of the DNA fragment of interest, and may be avoided using primers that are designed such that they lack complementarity—especially at the 3 ends—to itself or the other primer used in the reaction. If primer design is constrained by other factors and if primer-dimers do occur, methods to limit their formation may include optimisation of the MgCl concentration or increasing the annealing temperature in the PCR.