Spliceostatin B was also isolated fromPseudomonassp. due to its potential use for cancer treatment. Some microbial metabolites have shown the ability to inhibit gene splicing and the molecular mechanism responsible for this inhibition is being studied for future applications. Here, we summarize the main types of natural products that have been characterized as splicing inhibitors, the recent advances regarding molecular and cellular effects related to these molecules, and the applications reported so far in cancer therapeutics. 1. Introduction In eukaryotes, coding regions of the genome called exons are interrupted by noncoding sequences known as introns. During transcription, exons are identified while introns are removed from the immature mRNA (or pre-mRNA) to generate a mature and functional mRNA molecule. The mechanism responsible for this process corresponds to splicing and the machinery that performs this highly regulated event is the spliceosome, which is integrated by five small nuclear ribonucleoproteic particles (snRNPs) and more than 200 proteins that include auxiliary regulatory factors and components of other co- and posttranscriptional machineries . During splicing, a series of RNA-RNA, RNA-protein, and protein-protein interactions are responsible for the decisions that determine which sequences will be included in the mature transcript . Moreover, some sequences can be incorporated differentially into separated splicing events, leading to an increase in the coding potential of the genome by a process called alternative splicing. 2. Alternative Splicing and the Spliceosome The general splicing mechanism involves the recognition of exon/intron boundaries in a sequence-dependent manner. In mammals, the 5 end of the intron (5 splice site or 5ss) contains a characteristic TG, which recruits snRNP U1. On the opposite side, the 3 end of the intron (3ss) shows an invariant region called the branch point sequence (BPS), followed by a polypyrimidine-rich tract (pY-tract) and a conserved UNC0642 AG dinucleotide that indicates the end of the intron . The recognition of the 3ss involves the binding of SF1 to the BPS and the recruitment of the snRNP U2 auxiliary factor (U2AF) to the pY-tract and the AG dinucleotide. After the recognition of both UNC0642 exon/intron boundaries, an early complex is formed that commits pre-mRNA to undergoing splicing, where U2 snRNP is also recruited to the 3ss. U2 snRNP recruitment to the pre-mRNA is one of the key steps that triggers additional interactions, leading to the formation of catalytic spliceosome complexes due to the incorporation of the tri-snRNP U4/U5/U6 within which numerous RNA rearrangements and modifications in protein composition contribute to complete a splicing cycle [2, 3]. Like most of the snRNPs, U2 is a ribonucleoproteic complex formed by 7 Sm proteins (which are common for spliceosomal snRNPs) and 17 specific proteins, being the largest snRNP . Among the specific snRNP U2 components, two protein subcomplexes are found: SF3a and SF3b [3C5]. SF3a includes 3 subunits of 60, 66, and 120?kDa  while EIF2AK2 SF3b shows at least 8 specific subunits of 10, 14a, 14b, 49, 125, 130, 140, and 155?kDa . Components of the SF3a and SF3b UNC0642 subcomplexes bind to sequences in the pre-mRNA tethering U2 snRNP to the BPS and the 3ss. SF3b 155 is one of the most conserved subunits of U2 snRNP and it has shown the ability to bind splicing factors U2AF65 and p14 [3, 8]. Interestingly, this subunit has been related to the antiproliferative effect observed for some natural products that regulate the splicing mechanism and it results clear in the fact that targeting the spliceosome and modulating splice-site recognition could be relevant for the development of new therapeutic approaches, as will be further discussed. 3. The Role of Alternative Splicing in Human Disease Over the past 10 years, the role of alternative splicing in human disease has been growing. When the human genome project was completed,in silicoanalysis predicted that 75% of the human genes underwent splicing  and that 15 to 50% of the genetic diseases were related to aberrant splicing events . From this initial observation, several studies have linked splicing defects with specific genetic disorders. However, the full significance of the role in alternative splicing in human disease remains to be elucidated. Some diseases that have been linked to defects on splicing include dilated cardiomyopathy, autism spectrum disorder, spinal muscular atrophy, schizophrenia, cardiac hypertrophy, amyotrophic lateral sclerosis, and frontotemporal dementia . In all these cases, the molecular UNC0642 insights related to the splicing defect UNC0642 that originates the disease have been dissected. The precise regulation of the splicing event varies for each pre-mRNA and for this reason it is time consuming to demonstrate the molecular mechanism that regulates the alternative splicing for each gene. Moreover, this regulation also depends.