Nucleic Acids Research, 2005, Vol. 33, No. 7 2087 A53, which results in a frame shift. Since F411 and A030 show poor splicing activation compared with F011 and A016, respectively, these two subregions of F011 and A016 appear to serve as activation domains, presumably by interacting with other proteins. This notion is supported by the ﬁnding that the RRM-lacking isoform F402, which includes the same subregions II and IV as F011, functions apparently as a dominant-negative mutant to the wild-type Fxh, consistent with the interpretation that the mutant and wild-type are com- peting to interact with other protein(s). To date, Fyn tyrosine kinase and estrogen receptor-a have been reported to interact with Fxh, and ataxin-2 with A2BP1 (23,41,42). Whether these Figure 8. Amino acid sequence alignments of Fxh and A2BP1 isoforms. Shaded and striped boxes indicate identical and similar amino acids, respectively, to F011. RRM and the amino acids encoded by exons B40 and M43 (B40/M43) as well as the highly homologous regions between F011 and A016 (I–IV) are indicated.