Recipes for ULX formation: necessary ingredients and garnishments
  I summarize the main observational features that seem to recur more frequently in the ULX population. I speculate that two of the most important physical requirements for ULX formation are low metal abundance, and clustered star formation triggered by external processes such as molecular cloud collisions. In this scenario, most ULX are formed from recent stellar processes, have BH masses < 100 Msun and do not require merger processes in super star clusters. 
Roberto Soria (CfA/MSSL)