Cancer is like a car crash. Your body typically regulates the speed at which your cells divide,
癌症就像一场车祸。通常，你的身体调节着细胞分裂的速度，
but sometimes, cancer cuts the brake lines, and your cells divide too quickly,
但有时，癌症切断了刹车线，使你的细胞分裂速度过快，
accumulating mutations that cause them to veer away from their original function,
积累的变异使细胞偏离了它们原本的功能，
form dangerous tumors, and land you in the hospital.
从而形成危险的肿瘤，并把你送进了医院。
Cancer is basically an inability of the body to control the speed at which cells divide.
从根本上来说，癌症是身体缺乏调节细胞分裂速度能力的体现。
When cells divide too quickly, they can often accumulate mutations
细胞分裂过快就会积累变异，
that cause them to ignore their original function in the body, forming tumors.
从而导致细胞忽视它们在体内原本的功能，形成肿瘤。
In turn, these tumors may interfere with the natural processes of the body,
接下来，这些肿瘤可能会干扰身体的自然活动，
such as digestion and respiration, potentially leading to death.
比如消化和呼吸，并可能导致死亡。
Typically, your body has a number of genetic mechanisms to control how fast your cells divide.
通常来说，你的身体有一些控制细胞分裂速度的遗传机制。
One of these genes is BRCA1, which stands for breast cancer susceptibility gene 1.
其中一个叫做BRCA1，就是”乳腺癌1号基因”。
BRCA1 belongs to a class of genes called tumor suppressor genes.
乳腺癌1号基因是属于肿瘤抑制基因的一种。
Tumor suppressor genes are involved in regulating how fast a cell divides.
肿瘤抑制基因参与控制细胞分裂速度。
Normally, cell division follows an orderly process called the cell cycle, which is basically the life cycle of a cell.
一般来说，细胞分裂遵循一个叫做“细胞周期”的有序的进程，也就是一个细胞的生命周期。
Within the cell cycle is a series of checkpoints,
在细胞周期内有一系列的检验点，
where proteins, such as the one produced by BRCA1, regulate how fast the cell may proceed.
是诸如由乳腺癌1号基因制造的蛋白质调节细胞分裂速度的地方。

How does it do this? BRCA1 helps repair some forms of mutation in your DNA.
它是如何做到这些的呢？乳腺癌1号基因可以修复你的遗传物质中的某些变异。
If your DNA is damaged, BRCA1 keeps the cell from dividing until the mutation is repaired.
如果你的遗传物质遭到损坏，乳腺癌1号基因可以使细胞停止分裂，直到变异被修复。
You have two copies of the BRCA1 gene in every cell of your body.
在你身体的每个细胞内，乳腺癌1号基因有两个副本。
One copy you inherited from Mom, the other from Dad.
一个从妈妈那里遗传得来，另一个从爸爸那里得来。
This redundancy is a good a thing because you only need one functioning BRCA1 gene in a cell to regulate the cell cycle.
这种“多余”其实是件好事，因为你虽然只需要一个乳腺癌1号基因工作就能控制细胞周期。
But it's important to note that while these copies have a similar function they're not necessarily the same.
但有一点很重要：尽管这两个副本有着类似的功能，但它们并不是完全一模一样。
In fact, there are hundreds of variations, or alleles, of BRCA1.
事实上，它们之间有成百种差异，或者说，有成百种乳腺癌1号基因的等位基因。
Some regulate the cell cycle more effectively than others.
它们中的某些在调节细胞周期上比其他要更高效。
In other words, some people are born with better regulating and repair mechanisms than others.
换句话来说，一些人生来就有比别人更好的调节与修复机制。
And in some cases, mutations may render BRCA1 ineffective.
在某些情况下，变异可能导致乳腺癌1号基因失效。
When this happens, cells with damaged DNA are allowed to divide.
当这种情况发生，带有被损坏遗传物质的细胞会不受控制，继续分裂。
As they divide, these cells may accumulate additional mutations.
随着它们的分裂，这些细胞可能积累诱发更多变异。
These mutations may cause the cell to become less specialized and stop performing its original function in the tissue.
这些变异可能导致细胞失去功能，停止它们在人体组织内的正常工作。
If this occurs, then there's a greater chance they'll develop into cancer cells.
如果这种情况发生，它们就会有很大的几率演变成癌症细胞。
While we all have the gene, such as BRCA1, that can cause cancer,
我们每个人身体里都有诸如乳腺癌1号基因这类可能导致癌症的基因存在，
it's only when these genes fail at their function that problems develop.
但癌症只会发生在这些基因失效时。
Having an ineffective or mutated version of BRCA1 can increase your susceptibility to cancer,
带有失效或是变异的乳腺癌1号基因会增加你患癌症的风险，
much like driving with bad brakes increases the risk of an accident.
就像开一辆刹车坏掉的车子更容易发生事故一样。