The causative mutation for this disorder was discovered via the candidate gene approach. Tassi et al. (1984) reported a 10-15-fold decrease in the concentration of TG mRNA in affected cattle; Ricketts et al. (1985) reported the use of S1 nuclease assays and electron microscopy to narrow down the location of the mutation to the junction of exon 9 and intron 9; and Ricketts et al. (1987) reported the use of cloning and sequencing of genomic DNA from affected and normal animals to discover the causal mutation: a C>T transition creating a stop codon at position 697 in exon 9. Interestingly, both normal and affected cattle produce a second, shortened TG transcript that lacks exon 9. The sequences obtained from normal (European) and mutant (Afrikander) genes also differ by a missense mutation at codon 699 (A>G; Ala>Gly; a conservative amino acid change) and a A>G transition 25 bases into intron 9. It is not known whether these polymorphisms exist within the Afrikander breed. The intronic transition generates a Pst recognition site.
