S-acting regulatory factors to the pre-mRNA, and are thus required either to direct the splicing machinery to the appropriate sites or to inhibit the use of potential cryptic splice sites. ESEs, inparticular, appear to be widespread, and might be present in most, if not all, exons, including constitutive ones. The best characterized ESEs promote splicing 25033180 by Pleuromutilin interacting with members of the serine/arginine-rich (SR) protein family [4]. ESE motifs are quite degenerated and often overlapping, making them difficult to predict on the basis of the nucleotide sequence alone. For instance, analysis of SR-protein binding motifs showed that the major family members recognize fairly degenerated consensus sequences, varying from 5 to 7 nucleotides with a high purine content [5]. Silencing elements are less well characterized than ESEs, and their mechanisms of action are still not fully understood. The genetic context seems to be extremely important in determining the effect of both ESSs and ISSs. A well-established regulatory motif consisting of a stretch of three or more guanine nucleotides, the so called “G-run” element, may function both as an ESS and as an ISE, depending on its position. Indeed, it can cause exon skipping when placed within an exon, but it can also promote exon P7C3 web inclusion when located downstream of a weak 59 splice site [6]. Both ESSs and ISSs work by interacting with negative regulators,G-runs Regulating FGG Pseudoexon Inclusionwhich often belong to the heterogeneous nuclear ribonucleoprotein (hnRNP) family. In particular, the hnRNP I protein (also known as polypyrimidine-tract-binding protein, PTB) and proteins of the hnRNP A/B and hnRNP H families are among the bestcharacterized mediators of silencing [4]. Despite the efforts to classify general splicing regulatory sequences and their binding factors, exceptions are not uncommon: classical SR proteins are known to be involved in splicing repression in few cases [7], whereas some well-characterized hnRNP proteins may also act 
as splicing enhancers [8,9]. Therefore, experimental studies are required to clarify the role played by even well-known splicing factors in each specific gene context. Deciphering the splicing code is becoming increasingly important for the characterization of pathogenic mechanisms leading to human disease, as up to 60 of disease-causing mutations are found to affect splicing [10,11]. In general, changes in splicing cisregulatory elements can lead to exon skipping, intron retention, creation of ectopic splice sites, or activation of cryptic ones [12,13,14]. Another important pathological outcome of splicing mutations, which has been long overlooked, is the activation of pseudoexons. Despite the abundance of potential pseudoexons (50?00 nt-long intronic sequences 
with apparently viable splice sites at either end), their inclusion during normal pre-mRNA processing seems rare, although it has been described to occur as a regulatory mechanism for the expression of specific genes [15]. However, the actual frequency of pseudoexon activation might be underestimated due to nonsense-mediated-mRNA degradation of transcripts carrying out-of-frame pseudoexons. Most mutationinduced pseudoexon inclusion events originate from a single activating mutation, suggesting that many intronic sequences might be poised on the brink of becoming exons [16]. These mutations generally involve the creation of de novo functional donor or acceptor splice sites within an intronic sequ.S-acting regulatory factors to the pre-mRNA, and are thus required either to direct the splicing machinery to the appropriate sites or to inhibit the use of potential cryptic splice sites. ESEs, inparticular, appear to be widespread, and might be present in most, if not all, exons, including constitutive ones. The best characterized ESEs promote splicing 25033180 by interacting with members of the serine/arginine-rich (SR) protein family [4]. ESE motifs are quite degenerated and often overlapping, making them difficult to predict on the basis of the nucleotide sequence alone. For instance, analysis of SR-protein binding motifs showed that the major family members recognize fairly degenerated consensus sequences, varying from 5 to 7 nucleotides with a high purine content [5]. Silencing elements are less well characterized than ESEs, and their mechanisms of action are still not fully understood. The genetic context seems to be extremely important in determining the effect of both ESSs and ISSs. A well-established regulatory motif consisting of a stretch of three or more guanine nucleotides, the so called “G-run” element, may function both as an ESS and as an ISE, depending on its position. Indeed, it can cause exon skipping when placed within an exon, but it can also promote exon inclusion when located downstream of a weak 59 splice site [6]. Both ESSs and ISSs work by interacting with negative regulators,G-runs Regulating FGG Pseudoexon Inclusionwhich often belong to the heterogeneous nuclear ribonucleoprotein (hnRNP) family. In particular, the hnRNP I protein (also known as polypyrimidine-tract-binding protein, PTB) and proteins of the hnRNP A/B and hnRNP H families are among the bestcharacterized mediators of silencing [4]. Despite the efforts to classify general splicing regulatory sequences and their binding factors, exceptions are not uncommon: classical SR proteins are known to be involved in splicing repression in few cases [7], whereas some well-characterized hnRNP proteins may also act as splicing enhancers [8,9]. Therefore, experimental studies are required to clarify the role played by even well-known splicing factors in each specific gene context. Deciphering the splicing code is becoming increasingly important for the characterization of pathogenic mechanisms leading to human disease, as up to 60 of disease-causing mutations are found to affect splicing [10,11]. In general, changes in splicing cisregulatory elements can lead to exon skipping, intron retention, creation of ectopic splice sites, or activation of cryptic ones [12,13,14]. Another important pathological outcome of splicing mutations, which has been long overlooked, is the activation of pseudoexons. Despite the abundance of potential pseudoexons (50?00 nt-long intronic sequences with apparently viable splice sites at either end), their inclusion during normal pre-mRNA processing seems rare, although it has been described to occur as a regulatory mechanism for the expression of specific genes [15]. However, the actual frequency of pseudoexon activation might be underestimated due to nonsense-mediated-mRNA degradation of transcripts carrying out-of-frame pseudoexons. Most mutationinduced pseudoexon inclusion events originate from a single activating mutation, suggesting that many intronic sequences might be poised on the brink of becoming exons [16]. These mutations generally involve the creation of de novo functional donor or acceptor splice sites within an intronic sequ.
