 The different PDEs of the same family are functionally related despite the fact that their amino acid sequences show considerable divergence.The PDEs have different substrate specificities.Some are cAMP selective hydrolases (PDE 4, -7 and -8), others are cGMP selective hydrolases (PDE 5, -6 and -9) and the rest can hydrolyse both cAMP and cGMP (PDE1, -2, -3, -10 and -11).