Genetically engineered mice have been widely used to study gene function in a variety of life-science disciplines. However, the use of animal models in the field of nucleocytoplasmic transport has been limited, mainly because disruption of individual transport factors is expected to deregulate basic biological processes so severely that the embryo dies at an early stage in development. Early studies in which transport factors were knocked out in mice have confirmed this notion. Recent work has shown that hypomorphic alleles are very useful for studying essential genes at the organismal level. In combination with wild-type and knockout alleles, hypomorphic alleles can be used to generate a series of mice in which the expression of a protein is gradually reduced from normal to zero. Within this series, there is often an allelic combination that yields liveborn mice that develop overt phenotypes as they age, and that can be used to study the physiological relevance of the protein. In this article, we present an efficient method for generating an allelic series of mice. It involves the use of a multi-purpose gene-targeting vector that produces a hypomorphic allele that can also be converted into conditional and knockout alleles within the mouse. This method saves time and provides flexibility in terms of choosing the most appropriate model for studying components of the nucleocytoplasmic machinery at the organismal level.