Large DNA molecules (>l00 kb) are extremely sensitive to mechanical shearing m aqueous solution. Therefore, classical DNA extractron procedures from living tissues generally do not allow recovery of DNA fragments larger than 50‐100 kb m size. In recent years, technical improvements have made possible to purify and analyze large DNA molecules in vitro. Techniques such as pulsed‐field agarose gel electrophoresis (PFGE) (1 ) or cloning m yeast artificial chromosomes (YACs) (2 ) have been extremely useful in map‐based gene clonmg strategies. A large number of genes have been isolated by positional cloning in several model species, including plants like tomato and Arabidopsis (3 ). These megabase DNA techniques also allow initration of large‐scale physical mapping of complex genomes like Arabidopsis (4 ,5 ). Physical mapping of eukaryotic genomes generally involves genome reconstruction using ordered clones from large‐insert genomic libraries m YACs. Several Arabidopsis YAC libraries are available (6 -9 ).