A mutated form of arm, carrying point mutations (amino acid replacements E63A and E64A) in the acidic region of the N-terminus, is expressed under the control of its own promoter. The protein is tagged with a Tag:MYC epitope at residue 784 in the Gly/Pro-rich region close to the C-terminus. arm 3' trailer sequences are also present. The protein produced shows normal levels of binding to α-Cat protein and to shg protein in a coimmunoprecipitation assay.