These findings, together with the observations that the prolyl-tRNA synthetase-coding gene (26) and the transcription factor ATF4 (73) are overexpressed in different tumor types, and promote cancer metabolic homeostasis and survival (74) (Table 1), point to the key role of Proline availability in regulating protein synthesis, which sustains cancer cell proliferation and preserves stem cell identity. The gene discussed is ATF4; the disease is cancer.