Intracellular pools of Ca2+ serve as the source for inositol 1,4,5-trisphosphate (IP3) -induced alterations in cytoplasmic free Ca2+. In most human cells Ca2+ is stored in the lumen of the sarco/endoplastic reticulum by ATPases known as SERCAs (ATP2As). In platelets, ATP2As transport Ca2+ into the platelet dense tubular network. ATP2As are P-type ATPases, similar to the plasma membrane Na+ and Ca+-ATPases. Humans have three genes for SERCA pumps; ATP2A1-3. Studies on ATP2A1 suggest that it binds two Ca2+ ions from the cytoplasm and is subsequently phosphorylated at Asp351 before translocating Ca2+ into the SR lumen. There is a counter transport of two or possibly three protons ensuring partial charge balancing.