\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}{}%
  }}