subject/ 主语：The purpose of the work
Now, we have to answer, "which work?"
The work that Dr Lancaster, Dr Muotri and others are engaged in.
Other researchers who are involved in the field that Drs. Lancaster and Muotri work in.
verb/动词: has always been
complement/ 补语: to produce better laboratory models of neurological and psychiatric diseases
这个句子的主语是“工作的目的”。什么工作呢？Dr. Lancaster， Dr， Muotri， 和他们学术领域的别的研究人员做的那个工作。
Dr Lancaster, Dr Muotri 和他们的学术领域的别的研究人员做的工作的目的总是是发现更好的精神疾病模型.
Sorry for my bad Chinese.
It might be easier to look at a simpler example.
Some dogs like to run. 有一些狗喜欢跑。
Which dogs like to run?什么狗喜欢跑呢？
Dogs *that have eaten well and slept well* like to run. 刚才吃好了， 睡好了， 那样狗喜欢跑。
---> The purpose of the work is to produce models.
The purpose of which work is to produce models?
The purpose of the work *that Dr. Lancaster, Dr. Muotri, and others [(who are) involved] in the field are iengaged in* is to produce models.