A Chinese human genomic YAC (yeast artificial chromosome) library is being constructed with pJS97 and pJS98 as cloning vectors. An easier and more efficient protocol has been made successfully to protect the high molecular weight DNA from being degraded byusing polyamines solution and to remove smaller DNA by setting up a new method called "in situ electrophoretic size-fractionation". Furthermore, the glycerol at the final concentration of 20% is introduced directly into the medium used to grow the transformants, which would both simplify the operation and reduce the chance of contamination with other microorganisms or cross-contamination of YAC. clones. Such improved methods can be used in the construction and application of other genomic YAC libraries .