Appearance of a Brain‐specific Antigen (S‐100 Protein) in the Developing Rat Brain¹

Thelevel of the S‐100 protein, a brain‐specific antigen, wasdetermined by quantitative complement fixation in the brain stem and cerebrum of the rat during postnatal maturation. The content was minimal at birth in the brain stem and rose to its adult value by day 25. Although S‐100 protein could not be detected in the cerebrum of the 2‐day‐old rat, adult values were also present by the 25th day of age. Neither single dose X‐irradiation with 750 rd to the head at 2 days of age or single dose X‐irradiation at 11 days of age affected the adult level of S‐100 protein in the brain stem or cerebrum. Similarly, hypophysectomy at 20 days of age had no effect on the subsequent levels of S‐100 protein.