Covering YAC-Cloned DNA with Phages and Cosmids

The detailed analysis of the DNA cloned in yeast artificial chromosomes (YACs) is performed by subcloning into vectors such as phages or cosmids, which allow a simpler purification of insert DNA in addition to allowing high resolution mapping. Cosmids or phages are still a preferred DNA source for the isolation of new polymorphic markers or coding sequences. For example, the techniques used to isolate genes involve screening of cDNA libraries with whole cosmids or applying cDNA selection on immobilized cosmid DNA (1 ). Exon amplification is most effective when applied to cosmids (2 ). The combination of these resources has been instrumental in identifying disease genes like Huntington’s and Spinocerebellar Ataxia 1 (3 ,4 ). In order to generate cosmids or phages covering the YAC insert, two main strategies can be adopted:

1.	The screening of gridded chromosome-specific cosmid libraries with isolated, labeled YAC DNA (5 ,6 ) or Alu -PCR products.
2.	The construction of a library using whole YAC DNA (7 ). This method is useful in order to generate additional resources to the one just described or in cases where an ordered chromosome specific library is not available. Also, it can be used to fill in gaps in contigs formed in chromosome specific cosmid libraries.