In this note we shall show that the ground-state electron density \(\rho ({\mathbf r})\) is a functional of the highest occupied orbital in Kohn–Sham (Phys Rev 140:A1133, 1965, [1]) theory, \(\psi _{\mathrm {max}}\). The functionals \(\rho [\psi _{\mathrm {max}}]\) for an \((M+\delta )\)-electron system are resolved into three cases and connected to three Fukui functions defined by Parr and Yang (J Am Chem Soc, 106(14):4049, 1984, [2]).