A T to A point mutation in the 5' splice site of the STAT2 intron 4-5 results in the cryptic splicing of Stat2 mRNA by using a new 5' splice site GU located three nucleotides downstream of the mutation. The alternative splicing caused frame-shift and created stop codons which left out critical domains of STAT2 protein including DNA binding domain, Src homology 2 and transactivation domains. The mutation is hypomorphic as some wild-type protein is still detectable.