In contrast to the partial loss-of-function mutations in VHL that cause erythrocytosis in the homozygous state but retain sufficient HIF binding activity to suppress tumor formation, heterozygosity for a VHL loss-of-function mutation is not sufficient to cause erythrocytosis, but loss of the second allele in the tumor results in VHL activity that is insufficient to suppress tumor formation. This evidence concerns the gene VHL and neoplasm.