The paper by Alexander Altland and Martin R. Zirnbauer classifies mesoscopic normal-conducting-superconducting (NS) hybrid structures into four symmetry classes: $D$, $C$, $DIII$, and $CI$. These classes are identified based on the symmetry operations of time reversal and rotation of the electron's spin, corresponding to Cartan's symmetric spaces of type $C$, $CII$, $D$, and $DIII$. The authors focus on systems where the phase shift due to Andreev reflection averages to zero along a typical semiclassical single-electron trajectory, which are particularly interesting because they support quasiparticle states near the chemical potential without a genuine excitation gap. Disorder or dynamically generated chaos mixes these states, leading to novel forms of universal level statistics. For two of the four universality classes, the $n$-level correlation functions are calculated using a mapping to a free 1D Fermi gas with a boundary. The remaining two classes are related to Laguerre orthogonal and symplectic random-matrix ensembles. The weak localization correction to the conductance of a quantum dot with an NS geometry is calculated as a function of sticking probability and perturbations breaking time-reversal symmetry and spin-rotation invariance. Universal conductance fluctuations are computed from a maximum-entropy $S$-matrix ensemble, showing an enhancement by a factor of two compared to the naive expectation from analogy with normal-conducting systems. This enhancement is attributed to the doubling of the number of slow modes due to the coupling of particles and holes by the proximity to the superconductor. The paper also discusses the robustness of the symmetry conditions under Coulomb effects and provides a detailed analysis of the random-matrix ensembles and their implications for spectral statistics. The authors conclude that the symmetry conditions are robust and provide a quantitative description of the novel level statistics and transport properties in NS systems.The paper by Alexander Altland and Martin R. Zirnbauer classifies mesoscopic normal-conducting-superconducting (NS) hybrid structures into four symmetry classes: $D$, $C$, $DIII$, and $CI$. These classes are identified based on the symmetry operations of time reversal and rotation of the electron's spin, corresponding to Cartan's symmetric spaces of type $C$, $CII$, $D$, and $DIII$. The authors focus on systems where the phase shift due to Andreev reflection averages to zero along a typical semiclassical single-electron trajectory, which are particularly interesting because they support quasiparticle states near the chemical potential without a genuine excitation gap. Disorder or dynamically generated chaos mixes these states, leading to novel forms of universal level statistics. For two of the four universality classes, the $n$-level correlation functions are calculated using a mapping to a free 1D Fermi gas with a boundary. The remaining two classes are related to Laguerre orthogonal and symplectic random-matrix ensembles. The weak localization correction to the conductance of a quantum dot with an NS geometry is calculated as a function of sticking probability and perturbations breaking time-reversal symmetry and spin-rotation invariance. Universal conductance fluctuations are computed from a maximum-entropy $S$-matrix ensemble, showing an enhancement by a factor of two compared to the naive expectation from analogy with normal-conducting systems. This enhancement is attributed to the doubling of the number of slow modes due to the coupling of particles and holes by the proximity to the superconductor. The paper also discusses the robustness of the symmetry conditions under Coulomb effects and provides a detailed analysis of the random-matrix ensembles and their implications for spectral statistics. The authors conclude that the symmetry conditions are robust and provide a quantitative description of the novel level statistics and transport properties in NS systems.