  Although SU(2) isospin symmetry is generally assumed in the basic theory of
the strong interaction, a number of significant violations have been observed
in scattering and bound states of nucleons. Many of these violations can be
attributed to the electromagnetic interaction but the question of how much of
the violation is due to it remains open. A schematic model based on clustering
of quarks in the interior of the confinement region of the two-nucleon system
is introduced and evaluated. In this model the Coulomb interaction is the
source of all isospin breaking. It draws on a picture of the quark density
based on the diquark-quark model of hadron structure which has been
investigated by a number of groups. The model produces three isospin breaking
potentials connecting This illustrative model suggests that the breaking seen
in the low-energy NN interaction may be understood in terms of the Coulomb
force between members of diquark clusters. It allows the prediction of the
charge symmetry breaking interaction and the $nn$ scattering length from the
well measured $np$ singlet scattering length. Values of the $nn$ scattering
length around $-18$ fm are favored. Since the model is based on the quark
picture, it can be easily extended, in the SU(3) limit, to calculate isospin
breaking in the strange sector in the corresponding channels. A natural
consequence of isospin breaking from diquark clustering is that the breaking in
the strange sector, as measured by the separation energy difference between
$_{\Lambda}^4$H and $_{\Lambda}^4$He, is several times larger than that seen in
the comparison of three-nucleon mirror nuclei as observed experimentally.
