Many RNAs in nature have undergone post-transcriptional modifications, which play an important role in the structure, function, and metabolism of RNA. More than 170 types of RNA modifications have been discovered, among which pseudouridine modification is the earliest and most abundant RNA modification, so it is also called the "fifth base". Pseudouridine is the 5-position ribose isomer of uridine (see figure 1), which is widely present in a variety of RNAs (tRNA, rRNA, snRNA, snoRNA, etc.). Pseudouracil modification generally results from the isomerization of uridine. With the development of sequencing technology in recent years, it has been found that there are abundant pseudouridine modifications on mRNA. Pseudouridine is a natural structural analog of uridine. Compared with uridine, ribose is not connected to uracil N1, but to C5 of the pyrimidine ring. The isomerization of pseudouridine does not affect the classic base pairing, but it turns the N1 position into a proton donor. This may be because pseudouridine has different characteristics from other bases and can make many RNA structures stable.