For the circuit shown, assume that the NMOS transistor is in saturation. Its threshold voltage $V_{tn}=1V$ and its transconductance parameter $\mu_{n}C_{ox}\big(\frac{W}{L}\big)=1 \: mA/V^{2}$.Neglect channel length modulation and body bias effects. Under these conditions, the drain current $I_{D}$ in $mA$ is ________.