Infant acute leukaemia is characterised by specific genetic rearrangements and a rapid onset of disease shortly after birth. The vast majority of these cases bear rearranged MLL alleles. However, many facets of MLL-rearranged leukaemia are largely unknown. Basically, there exists a fundamental and evolutionary conserved relationship between the family of MLL/Trithorax proteins and the regulation of HOX gene clusters. Therefore, direct MLL fusion proteins are per se able to deregulate HOX genes, except when reciprocal MLL fusion proteins come into play. This reviews discusses (i) the current situation in MLL-rearranged leukaemia, (ii) the molecular and genetic tools to functionally investigate the many different MLL fusions, (iii) the latency of disease development, (iv) a novel cancer mechanism that has been recently uncovered when different MLL fusion protein complexes were characterized, (v) mutated signalling pathways in MLL-rearranged leukaemia and (vi) presents new ideas on how a given MLL fusion protein may modulate existing signalling pathways in leukaemic cells. The hypothesis is posed that the many different fusion partners of MLL are critically distinct entities for which specific inhibitors should be identified in the future.