The sex chromosomes of the plant species Silene latifolia (white campion) are very young (only 5-10My old), and all eleven X-linked genes so far described have Y-linked homologues. Divergence values between these gene pairs increases with their distance from the pseudoautosomal region, indicating a gradual or step-wise cessation of recombination between the X and Y. Theory predicts that X chromosomes should accumulate a non-random set of genes, due to the differential selection of X-linked genes in males and females. However, little is known about the importance of gene movements between the X and the autosomes in plants, or in any very young sex chromosome systems. Here, we isolate a new gene, Sl-Cyt, on the S. latifolia X, which encodes a cytochrome B protein. We show that Sl-Cyt moved from an autosome to the X very recently. It is now located only ~1cM from the pseudoautosomal region, i.e. in a region of the X that stopped recombining with the Y only recently. Genetic mapping in S. vulgaris suggests that Sl-Cyt originally belonged to a different linkage group from that of the other S. latifolia X-linked genes. In S. latifolia, there is no Y-linked homologue of Sl-Cyt, and also no autosomal paralogues seem to exist. Cyt is also X-linked in S. dioica, the sister species to S. latifolia, but is probably autosomal in S. diclinis, i.e. the translocation to the X probably occurred after the split between S. diclinis and S. latifolia/S.dioica. Diversity at Sl-Cyt is extremely low (πsyn = 0.16%), and we find an excess of high-frequency derived variants and a negative Tajima’s D, suggesting that the translocation was driven by selection.