In this talk we will present a recent result on the Gatto-Sartori-Tonin relation which is a functional relation between a mixing angle and one fermion mass ratio. We do a systematic study of it, finding by this, the necessary conditions to be met in order to obtain it within fermion mixing. We begin by considering two Dirac fermion families. From a mathematical viewpoint, it is found that a sufficient and necessary condition is to have a normal matrix with its first diagonal element equal to zero. Thereafter, this condition is translated into a phyisical statement which may also be interpreted as a symmetrical origin. Application of this statement to the three families case not only gives the desired relation but also explains why fermion mixing is decomposed into various two families mixing as recently observed by Hollik and Saldaña.