Heterozygous loss of the PU.1 locus in human AML. (A) Comparison of chromatograms obtained at diagnosis and in first remission in an AML-M2 patient (patient 26). Three informative SNPs within the URE, 2 SNPs within the promoter region (PROM), and 2 SNPs flanking exon 4 (1 each in intron 3, SNPi3; and in intron 4, SNPi4) showed heterozygosity at remission but LOH at diagnosis. The other SNPs (PRU2, PROM2, and PROM4) were not informative. (B) PU.1 mRNA expression of 107 (of 120) AML patients. Material was not available from 13 patients. The mean value for each AML subtype is presented as PU.1/ABL-ratio, with the value of AML-M0 patients being arbitrarily selected as 100%. Bars represent SD. The single line represents PU.1 expression of patient 26 with LOH in the PU.1 locus. BM indicates unsorted bone marrow of healthy persons; CD34, CD34⁺ selected cells of AML patients in remission. (C) FISH analysis of bone marrow at diagnosis of patient 26 with LOH in the PU.1 locus. The single red signal obtained from a probe directed against the URE sequences represents LOH of the URE, and the 2 green signals obtained from a probe against the centromeric region of chromosome 11 represent that indeed 2 chromosome 11 alleles are present.