Interestingly, AP3B1 levels varied greatly among patients with HPS type 2, with the highest level in a patient with a splicing site mutation near the 3' end of one allele (Patient #13 in Table 1) and the lowest level in a patient with biallelic nonsense mutations who had experienced an episode of hemophagocytic lymphohistiocytosis (Patient #12 in Table 1) (Fig. 4K), suggesting that HPS type 2 disease severity may correlate with residual AP3B1 expression. This evidence concerns the gene AP3B1 and hemophagocytic syndrome.