There are two things to look at. Some but not a
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Some but not all chemokines can bind to other receptor sites. Those whose main receptor is CCR5 do so inefficiently on other receptors, there are lesser amounts of other receptors and the reactions seem to be crippled. With the chemokines that are CCR5 specific and the wide variety of MOAs in action it shouldn't pose to much of a problem. Although I was surprised by 0 CTCs.
The second possible problem is tumor cell mutation. That tumor cells mutate at a much slower rate than viruses and we haven't seen resistance develop in HIV, it should vary from extremely rare to non-existent that this would occur.