INTERNATIONAL RULES OF NOMENCLATURE 519 



Appendix 2 of Standardized Nomenclature for Inbred Strains of Mice: Second 

 Listing 220 or Inbred Strains of Mice 628 to avoid duplication. Brief symbols are preferred. 

 An exception is allowed in the case of stocks already widely used and known by a 

 designation which does not conform. 



3. Definition of substrain. — The definition of substrain presents some of the same 

 problems as the definition of species. In practice, the determination of whether, 

 in published articles, substrain symbols should be added to the strain symbol, must 

 rest with the investigators using them. The following rules, however, may be of help. 



Any strains separated after eight to nineteen generations of b x s inbreeding and 

 maintained thereafter in the same laboratory without intercrossing for a further 12 or 

 more generations shall be regarded as substrains. It shall also be considered that 

 substrains have been constituted (1) if pairs from the parental strain (or substrain) 

 are transferred to another investigator, or (2) if detectable genetic differences become 

 established. 



4. Designation of substrains. — A substrain shall be known by the name of the parental 

 strain followed by a slant line and an appropriate substrain symbol. Substrain symbols 

 may be of two types. 



a. Abbreviated name as substrain symbol : The symbol for substrains should usually 

 consist of an abbrevation of the name of the person or laboratory maintaining it. 

 The initial letter of this symbol should be set in Roman capitals; all other letters should 

 be in lower case. Abbreviations should be brief, should as far as possible be stan- 

 dardized, and should be checked with published lists to avoid duplication. Examples: 

 A/He (Heston substrain of strain A), A/Icrc (Indian Cancer Research Centre substrain 

 of strain A). 



When a new substrain is created by transfer, the old symbol may be retained and a 

 new one added. Example: YBR/He, on transfer from Heston to Wilson, becomes 

 YBR/HeWi. The accumulation of substrain symbols in this fashion provides a history 

 of the strain. If the substrain symbols are not accumulated, the history of transfers 

 should be recorded in Inbred Strains of Mice. 



b. Numbers or lower-case letters: Numbers or lower-case letters may be used as 

 substrain symbols in certain circumstances. The position of these relative to other 

 parts, if any, of the substrain symbol should be suggestive of a historic or temporal 

 sequence. Thus, two substrain branches, separated in and maintained by one labora- 

 tory, may be designated by terminal numbers, with or without a preceding slant line. 

 Example: two sublines of A/HeCrgl, separated and maintained by Crgl, become 

 A/HeCrgl/1 (or A/Crgl/1) and A/HeCrgl/2 (or A/Crgl/2). Lower-case letters 

 immediately following the strain symbol, with a slant line only intervening, may be 

 employed when two substrains are separated from a common strain prior to complete 

 inbreeding. Example: C57BR/a and C57BR/cd. (These were separated after nine 

 generations of b x s.) The use of numbers or lower-case letters immediately after the 

 slant line, to designate lines separated after 20 or more generations b x s, is ordinarily 

 not recommended, but may occasionally be justified for sublines widely recognized 



