A ll jawed vertebrates have two classes of structurally and functionally distinct major histocompatibility complex (MHC) molecules that present peptides to T cell receptors, thereby initiating an adaptive immune response (1). Class I molecules are integral membrane glycoproteins made up of a heavy chain and ␤ 2 -microglobulin that present peptides derived from intracellularly synthesized proteins to CD8 ϩ T cells (2). On the other hand, class II molecules, consisting of two integral membrane glycoproteins designated ␣ and ␤ chains, present peptides produced in or directed to the endosomal͞lysosomal compartment to CD4 ϩ T cells (3). In most animals including human and mouse, class I genes (the genes for the heavy chain of class I molecules) are tightly linked to class II genes (the genes for the ␣ and ␤ chains of class II molecules), collectively forming a gene cluster (1,4,5).Although class I genes that play a major role in antigen presentation map to the MHC, several class I-like genes located outside the MHC have been identified in human and mouse. Examples of such class I-like genes in mice include Cd1 (Cd1d1 and Cd1d2) (6, 7), Azgp1 (the gene for zinc-␣2-glycoprotein) (8, 9), Fcgrt (the gene for the heavy chain of the neonatal IgG Fc receptor) (10, 11), Procr (the gene for the endothelial protein C receptor) (12), H2ls (the gene for the H2 complex class 1-like polypeptide) (13,14), Hfe (the hemochromatosis gene) (15, 16), Raet1a-Raet1e (the genes for retinoic acid early transcripts 1␣-) (17, 18), and H60 (histocompatibility 60) (19). Although these genes have diverse functions, some unrelated to immunity, they all have structural similarities to MHC class I genes and thus qualify as members of the class I gene family.We describe here a family of mouse class I-like genes located outside the MHC. This gene family, which maps to the vicinity of the leukocyte receptor complex (LRC) on chromosome 7, has two members. Interestingly, the proteins encoded by this gene family are related most closely to the MICA͞MICB molecules encoded in the human MHC. After consultation with the International Committee on Standardized Genetic Nomenclature for Mice, we propose to call this newly discovered gene family Mill (MHC class I-like located near the LRC) and its two members Mill1 and Mill2. Materials and MethodsIsolation of Mill1 and Mill2 cDNA by RT-PCRs. A computer search of the mouse EST section of the GenBank database resulted in the identification of three clones predicted to encode two distinct, class I heavy chain-like molecules. Based on the sequences of these clones, we designed primers that enabled the isolation of full coding sequences of Mill1 and Mill2 cDNA. The primer sequences were 5Ј-TGGCTTGGGATCTTCAAAGT-3Ј (sense) and 5Ј-TCCTCTGTCTTGTTTGGCTGTT-3Ј (antisense) for Mill1 and 5Ј-TTTGCACAAAACTCCATTTGA-3Ј (sense) and 5Ј-AGAAACAGCCAAGCCTCAGTC-3Ј (antisense) for Mill2. The cDNA templates for PCR were synthesized as described (20) by using total cellular RNAs isolated from the skin of 1-day-old BALB͞cCrSlc mice (Mill1)...