Alternative splicing is a major mechanism for regulating biological systems, producing multiple messenger RNA (mRNA) and protein isoforms. Some of these isoforms have distinct or even opposing functions (19) . Many genes in the immune system have been found to be alternatively spliced (20) (21) (22) and a growing number of human diseases are associated with aberrant splicing of the genes (23) (24) (25) . However, few studies to date have identified the mechanisms that regulate alternative splicing in the immune system. While CD200 exists as a single copy gene, data from Borriello et al. (26) , confirmed by our experiments (27) , have reported that a splice variant of CD200 exists. Although exon 2 deletion of CD200 caused by alternative splicing results in a frame shift and premature translational termination, we noted the existence of a downstream ATG start codon in a perfect Kozak context (27) . When the first start codon is followed shortly by a terminator codon and creates a small open reading frame (ORF; 5 0 -mini-cistron), the 40S ribosomal subunit remains bound to the mRNA, resumes scanning, and potentially reinitiates at the next ATG codon downstream (28) . It is known that the NH2-terminal region of CD200 is important for its biological interaction with CD200Rs (29, 30) , and translation from the second ATG start codon would produce a truncated form of CD200 (CD200 tr ) lacking the NH2-terminal 43 amino acids which includes regions important for the interaction with CD200Rs. Indeed, our previous studies have shown that expressed CD200 tr is a functional antagonist to CD200 (27) .