IMMUNOLOGY. For the article “Paternal monoallelic expression of the paired immunoglobulin-like receptors PIR-A and PIR-B” by Ching-Cheng Chen, Vincent Hurez, J. Scott Brockenbrough, Hiromi Kubagawa, and Max D. Cooper, which appeared in number 12, June 8, 1999, of Proc. Natl. Acad. Sci. USA (96, 6868–6872), the authors note the following correction. The authors have learned that the Jackson and Bailey substrains of inbred BALB/c mice have undergone sequence variation in their Pir genes over the 60 years of independent breeding. The allelic determinants recognized by the 10.4 anti-PIR antibody are expressed in the BALB/cBy substrain but are not present in the BALB/cJ PIR sequence (GenBank accession no. AF288356). Mixed provenance in the BALB/c parents for the [C57BL/6 (male) × BALB/cJ (female)]F₁ offspring versus [BALB/cBy (male) × C57BL/6 (female)]F₁ offspring described in the article led to the erroneous conclusion of monoallelic PIR expression. The data in this article are otherwise correct. The authors apologize for the inconvenience that their incorrect conclusion may have caused other investigators.