Background and purpose. Splice variants of P2X7 receptor transcripts contribute to the diversity of receptor-mediated responses. Here we investigated the expression and function of C-terminal truncated (ΔC) variants of the mP2X7 receptor, which are predicted to escape inactivation in one of the strain of P2X7-/- mice (Pfizer KO). Experimental approach. Expression in wild type (WT) and Pfizer KO tissue was investigated by RT-PCR and western blot analysis. The ΔC variants were also cloned and expressed in HEK293 cells to investigate their assembly, trafficking and function. Key results. RT-PCR indicates expression of a ΔC splice variant in brain, salivary gland and spleen from WT and Pfizer KO mice. An additional ΔC hybrid transcript, containing sequences of P2X7 upstream of exon 12, part of exon 13 followed in-frame by the sequence of the vector used to disrupt the P2X7 gene, was also identified in the KO mice. By blue native PAGE analysis and the use of cross linking reagents followed by SDS-PAGE, P2X7 trimers, dimers and monomers were detected in spleen and salivary gland of Pfizer KO mice. The molecular mass was reduced compared to P2X7 in WT mice tissue, consistent with a ΔC variant. Expressed in HEK293 cells the ΔC variants were inefficiently trafficked to the cell surface and agonist-evoked whole cell currents were small. Co-expressed with P2X7A, the ΔC splice variant acted in a dominant negative fashion to inhibit function. Conclusions and Implications. Pfizer KO mice are not null for P2X7 receptor expression but express ΔC variants with reduced function.