\usepackage{etoolbox} \newcommand\DefineSquence[2]{% \ifcsdef{DNA-#1}{% \GenericError{DNA Seq.}{Sequence key '#1' already defined}{}% }{% \csdef{DNA-#1}{#2}% }} \newcommand\DNA[1]{% \ifcsdef{DNA-#1}{\csuse{DNA-#1}}{% \GenericError{DNA Seq.}{Sequence key '#1' unknown}{}% }}