Correction

MEDICAL SCIENCES. For the article “Normal viability and altered pharmacokinetics in mice lacking mdr1-type (drug-transporting) P-glycoproteins,” by Alfred H. Schinkel, Ulrich Mayer, Els Wagenaar, Carla A. A. M. Mol, Liesbeth van Deemter, Jaap J. M. Smit, Martin A. van der Valk, Arie C. Voordouw, Hergen Spits, Olaf van Tellingen, J. Mark J. M. Zijlmans, Willem E. Fibbe, and Piet Borst, which appeared in issue 8, April 15, 1997, of Proc. Natl. Acad. Sci. USA (94, 4028–4033), the authors note the following correction. Fig. 1, and the first few lines of the Materials and Methods and Results sections, indicate that during gene disruption, exons 3 and 4 of the Mdr1b gene were deleted. However, there is an error in the map of the Mdr1b gene: exon 3 is positioned upstream (not downstream) of the first indicated NcoI site. As a consequence, only exon 4 of Mdr1b was deleted, and not exons 3 and 4. For the validity of the Mdr1b and Mdr1a/1b knockout mouse strains, this error has no consequences: deletion of exon 4 alone still results in a frameshift and still removes the first transmembrane segment plus some flanking sequences in Mdr1b. The absence of Mdr1b at the protein and functional level in the knockout mice also remains well documented. This correction, therefore, does not affect the conclusions of the paper.