Yeast colony size reflects YAC copy number.

A novel strategy for separation of co-cloned YACs was developed. For this, yeast cells were grown under non-selective conditions to allow the mitotic loss of multiple YACs. Yeast colonies of different size appear on 'drop-out' selection plates with small clones consistently containing a single-copy YAC. Different auxotrophic marker genes can be used to separate co-cloned YACs or reduce their copy number, which is essential for most YAC-modification procedures.