Buffering Mechanisms such as molecular titration can generate ultrasensitivity.  In vitro, this can be observed for the simple mechanism:
Where the monomeric form of A is active and it can be inactivated by binding B to form the heterodimer AB.  When the concentration of  ( = [B] + [AB]) is much greater than the , this system exhibits a threshold determined by the concentration of .   At concentrations of  ( = [A] +[AB]), lower than , B acts as a buffer to free A and nearly all A will be found as AB.  However, at the equivalence point, when  ≈ ,  can no longer buffer the increase in , so a small increase in  causes a large increase in A.  The strength of the ultrasensitivity of [A] to changes in  is determined by /.   Ultrasensitivity occurs when this ratio is greater than one and is increased as the ratio increases.  Above the equivalence point,   and A are again linearly related.  
In vivo, the synthesis of A and B  as well as  the degradation of all three components complicates generation of ultrasensitivity.  If the synthesis rates of A and B are equal this system still exhibits ultrasensitivity at the equivalence point.
One example of a buffering mechanism is protein sequestration, which is a common mechanism found in signalling and regulatory networks. In 2009, Buchler and Cross constructed a synthetic genetic network that was regulated by protein sequestration of a transcriptional activator by a dominant-negative inhibitor.  They showed that this system results in a flexibile ultrasensitive response in gene expression.  It is flexible in that the degree of ultrasensitivity can be altered by changing expression levels of the dominant-negative inhibitor. Figure 1 in their article illustrates how an active transcription factor can be sequestered by an inhibitor into the inactive complex AB that is unable to bind DNA. This type of mechanism results in an "all-or-none" response, or ultransensitivy, when the concentration of the regulatory protein increases to the point of depleting the inhibitor. Robust buffering against a response exists below this concentration threshold, and when it is reached any small increase in input is amplified into a large change in output.