This article reviewed recent studies on molecular stucture,classification,study methods,distribution,function,modulating factors and gene modulation, focus of study and also unresolved problems of NTT.

• 本文仅就近年来国外有关方面的研究概况，从分子结构、分型、研究方法、分布、功能、调节因素、基因调控、以及研究焦点和尚待解决的问题等诸方面做一综述。