Lynne:
I’m not a doctor, just a retired engineer with a background in research and a flair for using Google.

Normally we think of inherited defects when we think of chromosomal abnormalities , but this is not always the case.

Prompted by your question, I did some more digging and found that the villain of the piece is something called an oncogene:

An oncogene is a gene that, when mutated or expressed at high levels, helps turn a normal cell into a cancer cell.[wikipedia]

The Oncogenes may be activated to produce tumours in a variety of ways eg as a result of viral infection, chromosomal translocations, deletions and gene mutation.

The ph’ oncogene, which leads to chronic myeloid leukaemia, is believed to be caused by a chromosomal translocation, according to Leong Yuet Yow, a Consultant Pediatrician at Singapore General Hospital.

In his paper, he described in detail the geometry of the translocation, didn’t say what causes it in the first place, so I suppose more research is indicated.