The Jiangsu-based biotech firm said China's Customs Anti-Smuggling Department raided its offices in Nanjing and Zhenjiang, and its Chairman Fangliang Zhang is under "residential surveillance", while four other employees have been detained for questioning.
"The group's business operations remain normal. The company has taken legal advice as appropriate," Genscript said, adding it will assist authorities with the investigation, if required.
No charges has been filed against any entity or individual, the company said, adding that Zhang will continue to take part in decision making via appropriate means but is not participating in Genscript's daily operations.
Zhang is also chairman and chief executive officer of Genscript's unit Legend Biotech Corp. Legend Biotech has appointed its Chief Financial Officer Ying Huang as interim CEO, the Hong Kong-listed firm added.
Shares of Genscript plunged as much as 27.1% to HK$10.12, their lowest since January 2019, and on track for its worst day since September 2018. The benchmark Hang Seng slid 0.3%.
(Reporting by Donny Kwok; Editing by Kim Coghill)