The oncogene v-myb of the retroviruses AMV (avian myeloblastosis virus) and E26 (avian leukaemia virus) encodes a transcription factor (v-Myb) which is a truncated homolog of its cellular progenitor c-Myb. c-Myb plays an essential role in the development of haematopoietic cells and is known to be a regulator for many target genes. v-Myb AMV is responsible for the transformation of myelomonocytic cells and for arresting them in an immature stage, presumably because of a deregulation of the expression of specific target genes. In addition to the truncation of the coding region, a number of amino |