Inhibitory diffusible factor IDF45, a G₁ phase inhibitor

An inhibitory diffusible factor of 45 kDa (IDF45) was isolated from medium conditioned by dense cultures of 3T3 cells. The procedure involved Bio‐Gel PI 50 chromatography and 2 reverse‐phase FPLC. After the final step of purification, 60 of IDF45 inhibited 50% of α‐globulin‐stimulated DNA synthesis. It was shown that IDF45 acted in the G₁ phase of the cell cycle. When added for 8 h in the G₁ phase of the cell cycle, it was able to inhibit DNA synthesis in the S phase which followed this G₁ phase. Furthermore, IDF45 inhibited the early stimulation of RNA synthesis induced by α‐globulin.