In 2001 a 31-year-old white male (wt 70 kg) presented with acute lymphoblastic leukaemia in November 2000. Peripheral WBC was 82 x 109/L. The tumour was CD10+, CD13+, CD19+, CD20+, CD33+, CD34+, HLADR+, and TDT+. Standard cytogenetics failed, but cells were BCR/ABL negative by PCR. He was treated with an aggressive multiagent chemotherapy schedule (Northern Region Haematology Group NEALLVI) for a six-month period. An attempt at the end of therapy to harvest peripheral blood stem cells failed. There was no sibling donor or adult matched unrelated haemopoetic stem cell (HSC) donor available. Six months after completing the protocol he relapsed. Reinduction with further aggressive chemotherapy (FLAG-fludarabine, cytosine arabinoside and GCSF) was completed causing sustained and irrecoverable marrow hypoplasia with persistence of small numbers of leukaemia blasts. 
 With no adult HSC donor available we identified a fully HLA-matched male cord blood unit (total nucleated cell dose 1.4 x 107/kg, mononuclear cell dose 0.8 x 107/kg), CD 34 dose unavailable. This matched unit was blood group O, and the recipient was blood group A. This single unit was (in 2002 the date of transplant) considered insufficient to guarantee engraftment in a 70 kg recipient. After detailed discussions with patient and relatives, and considering preliminary data from Gryn et al., six additional totally HLA-mismatched, blood group matched, female cord blood units were selected from the Newcastle Cord Bank to increase the stem cell component. 
 Preconditioning consisted of Rituximab (anti-CD20 antibody) for four doses (325 mg/sq m per dose), over 8 days. Marrow blast content was unchanged directly after this treatment. Additional preconditioning consisted of intravenous Melphalan 120 mg/m2 plus TBI of 1200 cGy in six fractions over three days. Intrathecal methotrexate was also utilised. The HLA-matched cord unit was given prior to the nonmatched cord units to facilitate predominant engraftment with the matched element. The remaining cord units were all given over a one-hour period four days later. Nucleated cell content of these units is shown in Table 1.
Details of the single matched cord engraftment are tabulated in Table 2. White blood cell reconstitution showed a level of 1.3 x 109/L on day +28 and a staggering granulocyte count of 7.45 x 109/L on day +32 which reduced subsequently as GM CSF (started on day +7) was discontinued. All white cells were from the matched cord unit. Platelet engraftment, as anticipated, was slow and did not reach a level of 30 x 109/L unsupported until day 120 (Table 2). 
Molecular markers of the leukaemic clone (immunoglobulin heavy chain markers (IgH/VH1/VH2) and T cell gamma gene rearrangement (courtesy of Dr. L. Foroni, Royal Free Hospital, London) disappeared in clinical remission after chemotherapy, returning on subsequent relapse. These markers were no longer identifiable in the day 100 sample, post cord blood transplant. 
 Genotype tracking of all transfused mismatched cord blood units used was performed by VNTR analysis using three microsatellite loci (TNFa and TNFd (6p21.3) and the CTLA4 locus (2q33)). All transfused units had unique genotypes using these markers (Table 1). On Day +5 trace of the patient's genotype was detectable and subsequently became undetectable. The HLA-matched cord unit genotype was detected from day +7 with only weak signals from any of the nonmatched cord genotype between day's 5-7 after which they disappeared. (sensitivity of detection of this analysis is 2%-5 %). 
 In a separate study of mitochondrial mutations in haematological malignancy, the leukaemic cells from this patient were shown to have a somatic mitochondrial DNA mutation in the cytochrome b gene (He et al.). A sensitive specific PCR assay for detection of the mutation showed the mutation to be present at diagnosis and in relapse cells. After cord unit transplant there was no evidence of this mutation in the bone marrow, either at day +28, +100, or +140 indicating molecular remission at that stage (sensitivity of detection <0.1% using a quantitative SYBR Green real-time PCR assay, see He et al., for details). The day 100 bone marrow was populated only by the matched cord (FISH 100% chimaerism).