The PF4V1 gene comprised 3 exons and 2 introns. Exon 1 coded for a 34-amino acid hydrophobic leader sequence that had 70% sequence homology with the leader sequence of PF4. Exon 2 coded for a 42-amino acid segment that was 100% identical with the corresponding segment of the mature PF4 sequence. Exon 3 coded for the 28-residue carboxy-terminal region corresponding to a domain specifying heparin-binding and cellular chemotaxis. Human Gene Mapping Workshop 10.5 assigned the PF4V1 gene to 4q12-q21, the same region as that which carries the PF4, IP10, and MGSA genes. Green et al. (1989) suggested that the polymorphism detected in the PF4 gene by Guzzo et al. (1987) was in fact a polymorphism of the PF4V1 gene. Poncz et al. (1991) demonstrated that the PF4V1 and beta-thromboglobulin genes are located in a region of less than 190 kb.