Fusion of N-terminal MLL to heterologous Ras association domains from c-Raf1, RalGDS, or constitutively active mutants of K-Ras, H-Ras, or Rap1 does not activate the oncogenic potential of MLL. (A) Sequence alignment of the N-terminal regions of various RA domains. Dark shading defines residues of identity or similarity. Amino acid numbers are indicated on the left. The conserved secondary structures are depicted above, where arrows represent β sheets and the filled box represents an α helix. Crystal structures for c-Raf1, RalGDS, and Byr2 have been previously described.^36,38,41  Putative AF6 RA1 domain secondary structure was derived from the PHD prediction algorithm.³⁹  (B) Myeloid immortalization assay with various retroviral constructs encoding MLL fusions to heterologous RA domains or proteins are as indicated on the left. Each bar represents the mean ± SD of the total number of myeloid colonies per 10⁴ plated cells (≥ 4 replicates). (C) Western blot analysis of MLL fusion proteins expressed in transiently transfected virus-producing Phoenix cells.