A substantial quantity of data on Sprague-Dawley (SD) and Hannover Wistar rats 
strains have been published concerning their source, diet, and housing 
conditions, as well as the incidences of nonneoplastic lesions and neoplasms 
observed in different laboratories. Differences between the commonly used rat 
strains provided by different breeders (i.e., CD (SD) vs. Harlan Sprague-Dawley 
strain or Crl: WI(Han) vs. Wistar Hannover (Han)-derived strain, continued 
breeding by RCC Ltd., Switzerland, thereafter continued breeding by Harlan) may 
include, but are not limited to, body weight, incidence, and onset of major 
nonneoplastic lesions and neoplasms, and these can impact the development of a 
nonclinical safety program. Fisher 344 (F344) and SD rat strains generally have 
the highest tumor incidences, exceeding that in Wistar rats. Certain tumors are 
more commonly observed in one strain, and for some, the difference in incidence 
may be so significant that the tumor may even be considered characteristic for a 
specific strain (e.g., thymoma in Wistar and amphophilic renal adenoma in SD).