The methodologies of product line engineering emphasize proactive reuse to construct high-quality products more quickly that are less costly. Requirements engineering for software product families differs significantly from requirements engineering for single software products. The requirements for a product line are written for the group of systems as a whole, with requirements for individual systems specified by a delta or an increment to the generic set. Therefore, it is necessary to identify and explicitly denote the regions of commonality and points of variation at the requirements level. In this paper, we suggest a method of producing requirements that will be a core asset in the product line. We describe a process for developing domain requirements where commonality and variability in a domain are explicitly considered. A CASE environment, named DREAM, for managing commonality and variability analysis of domain requirements is also described. We also describe a case study for an e-travel system domain where we found that our approach to developing domain requirements based on commonality and variability analysis helped to produce domain requirements as a core asset for product lines.