  We outline the basic ideas involved in a recently proposed derivation of a
gauge theory for underdoped cuprates in the "spin-gap phase", performed
essentially step by step starting from the t-J model, considered as a model
Hamiltonian for the CuO_2 layers. The basic tool is the U(1)XSU(2) Chern-Simons
bosonization, to which it is dedicated a somewhat detailed discussion. The
basic output is a "spin-gap" not vanishing in any direction and an
antiferromagnetic correlation length proportional to the inverse square root of
doping concentration, in agreement with data deduced from the neutron
experiments. The model also exhibits a small half-pocket Fermi surface around
(\pi/2, \pi/2) and a linear in temperature dependence of in-plane resistivity
in certain temperature range.
