The postulate that a stalled/collapsed replication fork will be generated when the replication complex encounters a UV‐induced lesion in the template for leading‐strand DNA synthesis is based on the model of semi‐discontinuous DNA replication. A review of existing data indicates that the semi‐discontinuous DNA replication model is supported by data from in vitro studies, while the discontinuous DNA replication model is supported by in vivo studies in Escherichia coli. Until the question of whether DNA replicates discontinuously in one or both (...) strands is clearly resolved, any model building based on either one of the two DNA replication models should be treated with caution. BioEssays 27:633–636, 2005. © 2005 Wiley Periodicals, Inc. (shrink)