BHK-21细胞中的 21是什么意思~ 最好给明答案的来源

这个细胞株最早见于如下文献：
THE USE OF A LINE OF HAMSTER KIDNEY CELLS (BHK 21) FOR GROWTH OF ARTHROPOD-BORNE VIRUSES.
RF SELLERS
Trans R Soc Trop Med Hyg, Nov 1963; 57: 433-7.
其中提到“Syrian hamster kidney BHK-21 is a subclone (clone 13) of the parental line established from the kidneys of five unsexed, one-day-old hamsters in 1961; cells can be used for virus replication studies confirmed as hamster with IEF of AST, LDH, MDH, MPI, NP, PEP B”。