tissue sampling

It is talking about the calculation of a proportion/ratio:
For a certain gene locus （基因位点）, some mutations at it may be detected. If we want to calculate the proportion (or, rate of occurrence) of a certain mutant, we can divide the detected amount of the mutant by the total number of all types of the gene locus, i.e. "(mutated type + wild type)" （突变型+野生型）. 野生型 means those most commonly seen in samples/specimens and thus were taken as the normal type.

For 突变丰度（mutation abundance）, it is the amount of mutation(s) detected in a certain sample/specimen. If the specimen contains only a bit of tumor cells or the tumor cells in that rarely contain mutations, the mutation abundance of it will be low. So it is why the source text said "突变丰度受肿瘤细胞含量和取材等因素的影响" to readers.

I think 取材 here means 取材差异, referring to differences among specimens collected from different sites of a tumor, or through different sampling methods.

FYR

https://zhuanlan.zhihu.com/p/352321756
https://baike.baidu.com/item/野生型/8565714
https://www.qitaijk.cn/index.php/cms/show-1683.html


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Note added at 1小时 (2022-02-25 14:33:17 GMT)
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An explanation for different mutation abundances detected:

"不同的患者基因检测的突变丰度会有很大的差异，[...]同一份样本组织中，肿瘤细胞的基因、蛋白等各个结构并不一致。[...]
同一部位肿瘤会产生不同的基因表达，不同部位的肿瘤表达又有可能出现新的改变，[...]
也就造成了不同的患者即使具有相同的基因突变，彼此之间突变的丰度也会有所不同，进而表现为对于同一种药物的反应也不同。"

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Note added at 2天 21小时 (2022-02-28 09:59:09 GMT)
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1. “所占” lit. means "(the thing referred) occupies a proportion (in a larger amount)". Here it explains the (relative) proportion of a certain mutated type occurred in all types of the gene locus, both normal and mutated.
2. Yes, since the source text uses "即等于", lit. "(that is) equal to" to connect the two parts of the sentence, I think it is fine to translate it in a form of equation/formula. However its calculation is simply stated as "突变型/ (突变型+野生型)", and is not accurate enough, some extra clarification may need to be added ;)