Genomic fragment His3.3B which has been mutated to contain the amino acid replacement K4A.
Clones triply homozygous for His3.3AKO, His3.3BKO and Df(2L)BSC104 survive if they are carrying both His3.3BK4A and two copies of M{3xHisGU.H3.2.K4A}, although the clones are smaller than their wild-type twin spots.