DNA sequence analysis. (A) DNA sequencing of exon 5 of the fibrinogen Aα gene showed that II.1 and II.2, their brother (II.3), and his son (III.1) had a heterozygous C>T mutation that changed the CAA codon for residue Gln328 to a TAA stop codon and was the cause for the Keokuk truncation of the Aα constituent chain of the protein. (B) II.1 and II.2 and their mother (I.2) had a heterozygous mutation (IVS4 + 1 G>T) in intron 4 at the invariant 5′ splice site of this gene. Representative examples of each mutation are shown.