The FRT cassette present in P{His3.3B(FRT.His3.3B.WT.FLAG.ubiGFP)His3.3B.K4A.HA} has been excised (removing wild-type His3.3B sequences present within the cassette), allowing the mutated form of His3.3B (which contains the amino acid replacement K4A and which is also tagged at the C-terminal end with Tag:HA) to be expressed under the control of the His3.3B promoter.
His3.3BK4A.Tag:HA does not rescue the sterility of His3.3AKO His3.3BKO double homozygous males.