84 p. L. T. ILBEKY, P. A. MOORE, S. M. WINN, AND C. E. FORD 



for a conspicuously small chromosome, were raised for us by Dr. Mary Lyon 

 of the M.R.C. Radiobiological Research Unit, Harwell. The subsequent 

 hybrid from the T6 cross with C57BL has been designated C6. The cytological 

 examination was based on the hypotonic sodium citrate squash method (Ford 

 and Hamerton, 1956). 



The radiation source was a Theratron ^"Co Unit made available by Royal 

 Prince Alfred Hospital, Sydney, and the method of irradiation has been 

 previously described (Ilbery, 1960). 



EXPERIMENTAL 



The results of the following cytogenetic analysis of metaphase chromo- 

 somes from established radio-leukaemias illustrate the frequency of abnormal 

 karyotypes and serve as a basis for discussion in the pre-leukaemic experi- 

 ment described later. 



Phenomenon of abnormal chromosome complement frequently associated with 



radio-leukaemia 



C57BL, C6 and DBA/2 mice when 1 month old were subjected to four 

 doses of 180 rads whole-body ^''Co y-irradiation at four-daily intervals. All 

 leukaemias were of the typical radiation-induced thymic type either localized 

 or generalized. Either the propositus or the earliest possible passage was 

 examined cytogenetically. The presence of a clone was considered to have 

 been established in the leukaemic tissues when more than 5% of the cells so 

 examined carried a novel chromosome complement. Groups of cells possessing 

 a characteristic abnormal chromosome or a set of abnormal chromosomes are 

 inferred to be related by mitotic descent and are therefore referred to as 

 clones. The same inference cannot be rigorously applied to groups of cells 

 exhibiting abnormal counts whilst lacking any morphological change. 

 Nevertheless in this paper the term has been used in this extended sense for 

 this class or submode of cells. Individual abnormalities of form were recorded 

 if chromosomes that were clearly larger or shorter than any member of the 

 normal set were identified or if a metacentric chromosome or if a chromosome 

 with a prominent secondary constriction at an abnormal site were present. It 

 is now usual to refer to such chromosomes as markers. 



Of the 12 leukaemias observed in the propositus 8 had modes of cells 

 containing greater than 40 chromosomes. Counts could not be obtained from 

 the thymus in one because of technical failure but a mode of 43 was evident in 

 the haemopoietic tissues. The remaining 3 had modes of 40 but there were 

 classes of 42 in 2 and 39 in the third. Where spreading is good, whereas 

 clones greater than 40 are highly significant, counts of less than 40 chromo- 

 somes may be spurious because of breakage of cells during preparation. In 



