Cumulative historical record of concordant leukemia in twin children. Nk indicates subtype not known; single or 2 fused dots, twins diagnosed within 1 month of each other. Note: some reported cases with inadequate details have been omitted. First reported pair in 1882 are no. 1 in this sequence. References for this historical data set can be found at http://www.icr.ac.uk (mgreaves).