The Na + /Ca 2 + -K + exchanger (NCKX) utilizes the inward Na + gradient and the outward K + gradient to promote Ca 2 + extrusion from cells. Here, we have characterized a second NCKX from Drosophila. Based on its chromosomal location (X chromosome) we have named it Ncxk-x. Three splice variants were isolated with three distinct N-terminal sequences. NCKX-X differs from NCKX proteins described so far in other species by lacking an N-terminal signal peptide. Heterologous expression of the respective cDNA's resulted in NCKX-X protein expression and K + -dependent Na + /Ca 2 + exchange activity for two of the three splice variants. Transcript localization of Nckx-x was investigated and compared with that previously described by us for Nckx30C.