\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,amstext,amsopn,amscd,amsfonts}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
\frac{d[AiiA]}{dt} = k_1[mRNA]-k_{deg}[AiiA]
\end{displaymath}
\end{document}
