Unusual aberrant expression of a foreign gene in a particular transgenic mouse line is often attributed to chromosomal position effect, although proof of this is lacking. An alternative explanation is that expression has been modified by the arrangement of multiple copies of the foreign gene at the insertion site or by mutation or gene rearrangement. We have distinguished between these explanations in the case of one particular transgenic line by recovering the aberrantly expressed foreign DNA and reintroducing it into the mouse genome to produce secondary transgenic mice. The expression pattern of the gene in the secondary transgenic mice was normal, showing that this case of aberrant expression is due to a chromosomal position effect.