The Ashbya gossypiiEF-1α promoter of the ubiquitously used MX cassettes is toxic to Saccharomyces cerevisiae

► We examine the reported toxicity of TAP tagging proteins. ► TAP tagging does not generally cause loss of protein function. ► The MX marker cassettes are toxic to yeast at high copy numbers. ► Toxicity is due to the high copy presence of the heterologous TEF1 promoter. ► Genomic single copy integration of the MX marker and TEF1 promoter is not toxic.