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I was reading up Kaposi sarcoma and Robbins Pathology says,

..many features suggest that KS is not a malignant tumor despite the ominous name ...spindle cells in many KS lesions are polyclonal or oligoclonal

This is also seen in multinodular goiter where the lesions are polyclonal, but some monoclonal nodules are found.

My question is, how are these determined? By sequencing, is it not that, in cancerous growth there are myriad mutations, how can you map whether the mutations present are due to monoclonal expansion or polyclonal expansion? In non cancerous growths, wouldn't all cells have the same genotype?

Polyclonal and monoclonal expansion in B-cells is clear to me since a polyclonal lineage would have different vdj recombs while monoclonal would have the same. Here also, is it decided by genomic sequencing or the Ig sequence?

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There are a few ways to determine clonality. Sequencing-based methods are probably the cheapest, given the current costs of either sequencing, or the even cheaper PCR-based clonality assay. Other methods might include histomorphology, looking for known chromosomal aberrations and/or immunophenotyping, among others. Here's a nice review of some of the clonality testing methods.

Sequencing and PCR would be the most definitive ways to determine clonality of a tumor, since they provide information on the exact sequences of sets of cells.

Exome sequencing would be a good way to determine clonality in a large group of cells, where you don't necessarily know what to look for a priori. PCR is a lot cheaper and more focused and might make a better choice when you do have a specific target to sequence, such as highly polymorphic genes like some androgen receptors and the genes involved in VDJ recombination.

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