[en] Mutations in ilνO, the operator site for operon A of the ilν region of E. coli K-12, are reported to cause a cis dominant depression of the operon. A gene products and to map between ilνC and ilνA. Studies reported here demonstrate that the F25 episome which does not carry the region between ilνC and ilνA but only ilνE, ilνD and a part of ilνA, can transfer ilνO genetic material. Also, genes of operon A on the F25 episome respond normally to a derepression signal. It is proposed that ilνO is on the F25 episome. Five-point crosses demonstrate that mutations in ilνO map nearer to ilνE than to ilνA. Considering this and other evidence, it is proposed that the gene order in the ilν region is CADEO, not COADE, with transcription and translation in operon A being from ilνE to ilνA to ilνE. It is confirmed that mutations in ilνO cause a cis dominant depression of the operon A gene products but it is also noted that mutations in ilνO lead to the appearance of an AHAS activity more resistant to inhibition by valine than that of the present O + strain