Systematic Investigations of the Free Fermionic Heterotic String Gauge   Group Statistics: Layer 1 Results
  Using software under development at Baylor University, we explicitly construct all layer 1 gauge, weakly coupled free fermionic heterotic string models up to order 22 in four large space-time dimensions. The gauge models consist primarily of gauge content making a systematic construction process efficient. We present an overview of the model building procedure, redundancies in the process, methods used to reduce such redundancies and statistics regarding the occurrence of various combinations of gauge group factors and GUT groups. Statistics for both N=4 and N=0 models are presented. 
D. Moore, J. Greenwald, T. Renner, M. Robinson, C. Buescher, M. Janas,   G. Miller, S. Ruhnau, and G. Cleaver
hep-ph hep-th