Desmoplastic small round cell tumor t(11;22)
46,XX,t(11;22)(p13;q12)
Also known as: DSRCT, EWSR1-WT1, t(11;22)
46,XX,t(11;22)(p13;q12) is the translocation of desmoplastic small round cell tumor, fusing EWSR1 with WT1. It involves the same two chromosomes as Ewing sarcoma but a different arm of chromosome 11: q24 is Ewing, p13 is DSRCT, and the breakpoints are the only way to tell the two apart.
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What the notation means
46 is the number of chromosomes counted in the cell. That is the normal number: 22 pairs of autosomes, the chromosomes other than X and Y, plus one pair of sex chromosomes. XX lists the sex chromosomes, here two X chromosomes, the usual female set. These two fields open every karyotype because they describe the whole cell. Any abnormality is written after them.
A TRANSLOCATION (t): two chromosomes exchange segments. (11;22) lists the two chromosomes that exchanged segments, 11 and 22. The lower number is written first, or the sex chromosome if one is involved, so that the same translocation is always written the same way. (p13;q12) gives one breakpoint on each chromosome, in the same order as the chromosomes, so the breakpoints are 11p13 on 11 and 22q12 on 22. Each chromosome keeps its own centromere and takes the segment beyond the other's break.
Clinical notes
t(11;22)(p13;q12), desmoplastic small round cell tumour
EWSR1::WT1. On a karyotype this translocation means one disease: it is found in essentially every desmoplastic small round cell tumour and in nothing else, so the finding itself makes the diagnosis. The reading to guard against is Ewing sarcoma, which also joins chromosomes 11 and 22; the arm of 11 decides it, q24 and FLI1 for Ewing, p13 and WT1 here. The disease it announces affects adolescent and young adult males, spreads widely across the abdomen and pelvis, and carries a poor outlook. The same WT1, mutated in the germline, causes WAGR and Denys-Drash syndromes.