One hundred years has passed since the term of "gene" was coined in 1909, the gene definition has been revised many times in the past 100 years. Gene has changed from an abstract symbol to a specific segment, which can produce protein or functional RNA, and finally became one of the most important biological words. With the accomplishment of the genome project, particularly the project of Encyclopedia of DNA Elements (ENCODE), our knowledge about the the complexity and diversity of genomic organization and dynamics of genomes posed important challenges to the classical molecular gene concept. As is becoming more evident that the relations between information stored at DNA level and functional products are very intricate, some people consider that it was time to make a redefinition of gene. In this review, we briefly outline gene definition of this history and the development of gene concept these days.