New studies have shown that recent dog breeding practices among Cavalier King Charles Spaniel breeders has caused gene mutations linked to myxomatous mitral valve disease, or MMVD for short. In this condition, the mitral valve in the heart degenerates, allowing blood to leak from the left ventricle back into the left atrium. Researchers are looking for the variants that cause this mutation, and have, so far, been able to identify two. These genetic mutations appear to regulate a gene that codes for a common protein in heart muscle. Researchers have speculated that years of breeding have caused these mutations. In the 1600's, cavaliers were a very popular breed amongst royalty, and according to researchers, experienced several "bottlenecks" throughout their history. Bottlenecks are explained to be where only a small percentage of individuals were used for breeding and passed on their genes, so the hypothesis is that these individuals possessed the mutations or made the harmful genes for MMVD through centuries of selective breeding.