Observing proteins structural changes at during function in-side the cell is a challenge yet to be met. In this context we have been developing in-cell 95 GHz DEER ( double- electron electron resonance) distance measurement using Gd(III) spin labels. The developments that enabled in cell DEER on proteins, including instrumental aspects, Gd(III) spectroscopic properties, the chemistry of the Gd(III) labels and measurement methodology, will be first presented. Next, recent results showing that the monomer- dimer equilibrium constant of a protein is different in vitro and in the cell are different. Similarly, the protein conformation may also be different in these two very different environments.