(Perler, F. B. (2002). InBase, the Intein Database. Nucleic Acids Res. 30, 383-384)
Several conserved motifs have been observed by comparing intein amino acid sequences. There are two nomenclatures for these motifs: Blocks A, B, C, D, E, H, F, G or Blocks N1, N3, EN1, EN2, EN3, EN4, C2 and C1, respectively . Blocks N2 and N4 are not as well conserved as the other intein motifs. The intein motifs are more extensively described in the Conserved Intein Features - Do you Have an Intein? section and their sequences are listed in the Splicing Motifs and LAGLIDADG (DOD) Homing Endonuclease Motifs sections

The consensus sequence for each block is indicated below. Although, no single residue is invariant, the Ser and Cys in Block A, the His in Block B, the His, Asn and Ser/Cys/Thr in Block G are the most conserved residues in the splicing motifs. Any member of an amino acid group may be present in the remaining positions, even when a specific predominant residue is indicated
(Perler, F. B. (2002). InBase, the Intein Database. Nucleic Acids Res. 30, 383-384).

Remember, the splicing domain consists of Blocks A, N2, B, N3, F and G, while Blocks C, D, E and H are only in the DOD homing endonuclease domains. Blocks C and E each contains an endonuclease active site Asp (D) or Glu (E). Block D contains an endonuclease active site Lys (K) in the Sce VMA intein (Duan 1997). Several inteins have mutations in these endonuclease active site residues and therefore may not be active endonucleases although the remainder of the motif is present.
Key: upper case letters represent the standard single letter amino acid code for the most common amino acid at this position and lower case letters represent amino acid groups: ., any residue; h, hydrophobic residues: G,A,V,L,I, M; p, polar residues: S,C,T; a, acidic residues: D or E; r, aromatic residues: F,Y,W)

(Francine B. Perler: InBase: the Intein Database: Identifying Inteins by Conserved Intein Features Vol. 30, No. 1 383-384, Research, 2002)

A. Splicing Motifs (Blocks A, B, F, G)

N-terminal (N) domain
	<--> 1-33 AA		<--> 16-75 AA		<--> 3-37 AA
N1		N2		N3		N4

The first motif is found at the N-termini of inteins. Note the conserved Ser and Cys at the N' intein splice site. The OH/SH side groups of the aa in this position are necessary for the N-O/S shift of the peptide bond to the N-extein. The His at position 10 of the third motif is the most conserved intein residue. It was first predicted and then shown to be involved in the protein splicing reaction. Intein structures also showed this residue to be positioned at the protein splicing active site.

C-terminal (C) domain
	<--> 1-9 AA
C2	C1

The two C domain motifs are involved in the final steps of protein splicing. A Ser, Thr or Cys are found at the N-terminus of the C-extein (last position of second motif). The hydroxyl, or thiol, group of this aa attacks the last aa of the N-extein in a transesterification reaction. The resulting branched intermediate is resolved by the cyclization of the Asn preceding the attacking aa. This Asn, the intein's C-termini, is the second most conserved residue in inteins. Gln residues are now also known to occur in this position. They are found in the CIV RIR1 and Pho polC inteins. These inteins are integrated in conserved protein regions of vital proteins (subunits of ribonucleotide reductase and replicative DNA polymerases). Hence it is likely that these inteins are active and capable of protein splicing. The splicing reaction is suggested to be a variation of the Asn cyclization in which the Gln will undergo cyclization to glutarimide ring.