[长难句突破]

The exact mechanisms involved are still mysterious, but the likelihood that many cancers are initiated at the level of genes suggests that we will never prevent all cancers.

主体句式：The exact mechanisms are still mysterious, but the likelihood are initiated at the level of...

结构分析：本句由but连接的两个部分组成，是转折的关系。前半句中involved是用来形容主语的定语。后半句的主干是the likelihood suggests that...，其中主语后面跟了一个that引导的同位语从句，谓语后面则是一个that引导的宾语从句。

[全文译文]

"I have great confidence that by the end of the decade we'll know in vast detail how cancer cells arise," says microbiologist Robert Weinberg, an expert on cancer. "But," he cautions, "some people have the idea that once one understands the causes, the cure will rapidly follow. Consider Pasteur. He discovered the causes of many kinds of infections, but it was fifty or sixty years before cures were available."

This year, 50 percent of the 910,000 people who suffer from cancer will survive at least five years. In the year 2000, the National Cancer Institute estimates, that figure will be 75 percent. For some skin cancers, the five-year survival rate is as high as 90 percent. But other survival statistics are still discouraging-----13 percent for lung cancer, and 2 percent for cancer of the pancreas (胰腺).

With as many as 120 varieties in existence, discovering how cancer works is not easy. The researchers made great progress in the early 1970s, when they discovered that oncogenes, which are cancer-causing genes (基因), are inactive in normal cells. Anything from cosmic rays to radiation to diet may activate a dormant oncogene, but how remains unknown. If several oncogenes are driven into action, the cell, unable to turn them off, becomes cancerous.

The exact mechanisms involved are still mysterious, but the likelihood that many cancers are initiated at the level of genes suggests that we will never prevent all cancers. "Changes are a normal part of the evolutionary process," says oncologist William Haywar. Environmental factors can never be totally eliminated; as Hayward points out, "We can't prepare a medicine against cosmic rays."

The prospects for cure, though still distant, are brighter. "First, we need to understand how the normal cell controls itself. Second, we have to determine whether there are a limited number of genes in cells which are always responsible for at least part of the trouble. If we can understand how cancer works, we can counteract its action. "

"我有极大的信心相信到这个十年期结束时我们将会详尽地知晓癌细胞的生成原因，"微生物学家罗伯特·温伯格，一位癌症专家如此说道，"但是，"他又告诫说，"有些人认为一旦人们弄清了病因，治疗方法很快就会跟上。其实不然，想想法国细菌学家巴斯德，他发现了许多传染病的成因，但治疗方法却在五六十年后才问世。"

今年，91万癌症患者中有一半将至少再活5年。据国家癌症研究所，到2000年存活率将升至75％。现在对于一些皮肤癌来说，5年期存活率高达90％，但其他类型癌症的存活数据却仍令人沮丧--肺癌13％，胰腺癌2％。

癌症现有种类多达120多种，发现其规律实属不易。研究人员在70年代早期取得了很大进展，他们发现导致癌症的癌基因在正常细胞里是不活跃的。从宇宙射线、辐射到日常饮食，任何东西都可能激活一个处于静止状态的致癌基因，但如何激活的原因却尚不为人知。如果若干癌基因被激活，细胞无力排除也就演变成了癌细胞。

导致癌变的确切机制仍是个谜，但许多癌症始于基因这一可能性表明我们将永远不能预防所有癌症。"变化是进化过程中的正常部分，"肿瘤专家威廉·海沃德说道。环境因素永远无法完全消除，正如海沃德所指出的那样："我们无法准备出一种抗宇宙射线的药。"

治癌前景尽管遥远，但已越来越明朗。

"首先，我们需要了解正常细胞如何控制自己。其次，我们必须查明细胞中是否有一定数量的基因对至少是部分的麻烦总是负有责任。如果我们能弄清癌症的活动规律，我们就能采取相应的措施。"