Blue Colony?

[ddsyr at NS.EAST.CN.NET]

Dear Netters:
Recently I tansformed the pGEM-3Z plasmid vector (from Promega) into E. coli DH5a and 
spreaded on LB plate containing amp 100 ug/ml, X-Gal 20 ul (4% )and 8 ul IPTG(2%). After 
37C overnight, I found the some colonys on plate are blue, but more than half of them 
are white. Since I only used the vector, this phenomeno should not happen. Can anynoe 
give me suggestion? Thanks in advance.
Ma