A novel cell-free protein synthesis system called eCells produces amino acids based on specific isotopes using low-cost precursors. The system selectively labels methyl groups, i.e valine and leucine, with high efficiency. eCells achieve high levels of ¹³C incorporation and deuteration in protein preparations, making them suitable for NMR experiments of large protein complexes. They are easy to prepare, can be scaled up in volume and are a promising tool for protein production and NMR studies.