Sequence encompassing exons 5, 6, and 7 was replaced by PGK-neo inserted by homologous recombination. The deleted region encoded the enzymatically active extracellular portion of the mature protein that included apyrase-conserved regions 2 through 4. Although transcript 3' of the neo insertion was detected in homozygous mutant mice Northern blot analysis, apyrase activity and immunoreactivity were undetected.