A notable feature of eukaryotic nuclear pre-mRNA introns is the relatively high level of conservation of the primary sequences of 5 and 3 splice sites over a great range of organisms.
Between 1989 and 1991, several groups reported four independent examples of introns with a splice site that differed from the common intron:
*Cartilage matrix protein (CMP/MATN1) gene in humans and chickens
*Proliferating cell nucleolar protein P120 (NOL1) gene in humans
*Mouse Rep3 gene, presumably involved in DNA repair
*Drosophila prospero gene that encodes for a homeobox protein
In 1991 by comparing the intron sequences of P120 and CMP genes, IJ Jackson reported the existence of ATATCC (5) and YYCAC (3) splice sites in these introns. The finding indicated a possible novel splicing mechanism. 
In 1994, S.L. Hall and R.A. Padgett compared the primary sequence of all reports on the four genes mentioned above. The results suggested a new type of introns with ATATCCTT 5 splice sites and YCCAC 3 splice sites and an almost invariant TCCTTAAC sequence near the 3 end of the introns (so called 3 upstream element). A search for small nuclear RNA sequences that are complementary to these splice sites suggested U12 snRNA (matches the 3 sequence) and U11 snRNA (matches the 5 sequence) as being putative factors involved in splicing of this new type of introns.   
In all these four genes, the pre-mRNA contains other introns whose sequences conform to those of major class introns. Neither the size nor the position of the AT–AC intron within the host gene is conserved.
In 1996, Woan-Yuh Tarn and Joan A. Steitz described an in vitro system that splices a pre-mRNA substrate containing an AT–AC intron derived from the human P120 gene. Psoralen cross-linking confirms the base-pairing interaction predicted by Hall and Padgett between the branch site of the pre-mRNA substrate and U12 RNA. Native gel electrophoresis reveals that U11, U12, and U5 snRNPs assemble onto the P120 pre-mRNA to form splicing complexes.