Spir proteins nucleate actin polymerization by assembling a linear actin oligomer along a cluster of four actin-binding WH2 domains, and this process is enhanced by formins of the Cappuccino family.

Both migration and phagocytosis require actin polymerization, and unsurprisingly, HEM1-deficient neutrophils showed decreased actin polymerization and a diffuse distribution of F-actin.