These effects are achieved in part by downregulation of multiple phosphatases, which leads to high levels of steadystate phosphorylated intermediates and reducing the threshold of T cell receptor signaling.
Some thymocytes are able to rescue failed positive selection by receptor editing (rearrangement of the other T cell receptor allele to produce a new T cell receptor).
Unlike most genes, which have a stable sequence in each cell which expresses them, the T cell receptor is made up of a series of alternative gene fragments.