G.G., a 5 yo girl who has been followed-up from birth due to genetically proven Nijmegen syndrome. Last blood test before leukemia onset had been in September, 2016: WBC 4.7, ly 43%, minimal thrombocytopenia (176, age norm 189-394), low T4 cells (0.27, age norm 0.5-2.4), low IgG (2.72 g/L, age norm 5-15). Everything else OK.  
06.12.16. After a short prodrome she developed fever, hepatosplenomegaly (liver +5cm, spleen +6cm) with multiple small mesentric nodes. Blood showed leukocytosis up to 90.000 with>90% atypical cells (photos marked d00…), anemia and thrombocytopenia, LDH 2,000.  
There were medium-sized atypical pleomorphic cells with partial maturation. Flow showed mature T phenotype: cytCD3+, CD7++, mCD3++, CD45++, TdT-, CD34-, CD5++, CD2+, CD1a-, CD99-, CD10-, CD8++, CD4 very weak, TCRab+, CD56-. CD38++, CD25 weak, CD16+, CD45RA-, HLA-DR weak.  
Cytogenetics showed a mixture of multiple hyperploid clones.  
TCR/IG clonality was performed in Lithuania; no clonal TCRB, TCRG, TCRD rearrangements detected, polyclonal background also absent. IGH - a limited diversity of polyclonal/oligoclonal pattern, IGK, and IGL in germline status.  
Marrow was totally infiltrated by phenotipically the same cells, Granzyme B was negative.  
The initial tentative diagnosis was mature T-ALL. Reduced ALL BFM protocol (50% dose at the beginning, later increased to 75%) commenced; therapy was tolerated well. Clinical status improved with some decrease in hepatosplenomegaly and LDH. Only partial remission was achieved at control points (see marrows at day 33 and day 78).  
EBV antibodies were negative, there had been documented CMV infection earlier. CMV copy number rose during treatment: December 16 – 510, - December 21 – 8950 (therapy with IVIG+Acyclovir), December 28 - 537, January 4, 17, 31 - negative.  
After some consulting the case was considered to be the systemic T-cell lymphoproliferative disease of childhood, but CMV-driven in immunodeficiency setting.  
On January, 30 at admission for the next course: substantial hepatosplenomegaly, multiple 15-20mm intraabdominal nodes, PLT decreased from previously normal range and LDH increased to 1055 U/l.  
Marrow from February, 3 (photos marked 4…). Incomplete remission of T-LPD. In addition, there is massive myeloid hyperplasia with eosinophilia, disorganised, but with more or less preserved maturation and no blasts. Increased atypical megas, including microforms. Deep reduction of erythropoiesis.  
There have always been problems with marrow cytology (prominent hemodilution), probably due to prominent myelopathy; trephines were much more sensitive for MRD. The aspirate taken simultaneously with the last trephine was deeply hypocellular, too, but WBC were higher than in blood (4.000 vs 0.8), there was some atypia - see photos marked cyt... I am particularly proud of finding a micromegakaryocyte.  
Blood values at the moment: WBC 0.79, NEU 0.4, LYM 0.3, HGB 9.4 (after transfusions), MCV 88.1 (N < 85), MCH 28.7 (N < 28.6), normoblasts 1.3%, PLT 71 (after transfusions).  
I've never seen such regeneration in ALL kids. Taking into account Nijmegen's fragile genome, could it be a secondary MDS?