  Let $G$ be the simple group ${\rm PSU}(2,2^{2^n})$, $n>0$. For any subgroup
$H$ of $G$, we compute the M\"obius function $\mu_L(H,G)$ of $H$ in the
subgroup lattice $L$ of $G$, and the M\"obius function $\mu_{\bar L}([H],[G])$
of $[H]$ in the poset $\bar L$ of conjugacy classes of subgroups of $G$. For
any prime $p$, we provide the Euler characteristic of the order complex of the
poset of $p$-subgroups of $G$.
