A rather low concordance of childhood ALL in monozygotic twins would be consistent with this hypothesis (Ford et al, 1998), as would the demonstration of identical TEL-AML1 fusion genes in a set of monozygotic twins diagnosed with t(12;21) B-precursor ALL at ages 9 and 14 years (Wiemels et al, 1999b) as well as the simultaneous existence of identical TEL-AML1 fusion genes and non-identical deletions of the TEL gene, also believed to play a role in leukaemia development, in a pair of monozygotic twins concordant for t(12;21) positive ALL (Maia et al, 2001). Here, RUNX1 is linked to leukemia.