  We study general (not necessarily Hamiltonian) first-order symmetric systems
$J y'(t)-B(t)y(t)=\D(t) f(t)$ on an interval $\cI=[a,b> $ with the regular
endpoint $a$. It is assumed that the deficiency indices $n_\pm(\Tmi)$ of the
minimal relation $\Tmi$ in $\LI$ satisfy $n_-(\Tmi)\leq n_+(\Tmi)$. By using a
Nevanlinna boundary parameter $\tau=\tau(\l)$ at the singular endpoint $b$ we
define self-adjoint and $\l$-depending Nevanlinna boundary conditions which are
analogs of separated self-adjoint boundary conditions for Hamiltonian systems.
With a boundary value problem involving such conditions we associate the
$m$-function $m(\cd)$, which is an analog of the Titchmarsh-Weyl coefficient
for the Hamiltonian system. By using $m$-function we obtain the Fourier
transform $V:\LI\to L^2(\Si)$ with the spectral function $\Si(\cd)$ of the
minimally possible dimension. If $V$ is an isometry, then the (exit space)
self-adjoint extension $\wt T$ of $\Tmi$ induced by the boundary problem is
unitarily equivalent to the multiplication operator in $L^2(\Si)$; hence the
spectrum of $\wt T$ is defined by the spectral function $\Si(\cd)$. We show
that all the objects of the boundary problem are determined by the parameter
$\tau$, which enables us to parametrize all spectral function $\Si(\cd) $
immediately in terms of $\tau$. Similar results for various classes of boundary
problems were obtained by Kac and Krein, Fulton, Hinton and Shaw and other
authors.
