What are the symptoms? Jacobsen syndrome is associated with a recognisable pattern of features. The children are usually of a pleasant disposition with a characteristic face, somewhat more pear shaped than normal. There may also be some developmental delay (learning difficulties, see entry Learning Disability), varying from mild to more severe. Speech and language may be slower. However, heart problems such as enlarged left heart syndrome (see entry Heart Defects) can be more debilitating, sometimes requiring surgery. Blood disorders, mainly in the form of easy bruising and prolonged bleeding due to low numbers of platelets (thrombocytopenia – reduction in the number of platelets present in the blood and referred to as Paris-Trousseau syndrome) are common. Gastrointestinal problems including pyloric stenosis (a narrowing of the outlet from the stomach to the small intestine) and frequent respiratory problems also are seen. Individuals with Jacobsen syndrome may show some or all of these features although there is great variability in the number and severity of symptoms. The life expectancy for individuals with Jacobsen syndrome currently remains unknown but is increasing as we understand more about the symptoms and how to manage them. The two most common causes of illness and death are congenital heart defects and bleeding. The improved outcomes in children with most forms of congenital heart disease, however, suggest that the outcomes for children with Jacobsen syndrome with congenital heart defects is also likely to continue to improve. What are the causes? It is caused by the loss of a small portion of a chromosome at conception. A chromosome is made up of DNA code in a thread-like structure present in the nucleus of most cells in the body. The DNA code in turn carries genes which are the all important instructions about how each cell in the body should divide, grow and die. Genes are ‘strung’ along chromosomes rather like beads are strung along a necklace. In each cell, there are 23 pairs of chromosomes. The body needs a full compliment of chromosomes for normal health and development. Sperm and ova (‘egg cells’) carry one representative of each chromosome and at fertilisation usually fuse to create a baby with the full number of chromosomes shared equally from both parents. In Jacobsen syndrome, a small piece of the 11th chromosome is deleted (or missing) and this causes a range of clinical features in individuals with this condition. The features are related to the genes that are absent from the small piece of chromosome 11 that is missing and the structure of the chromosome shows a fragility that causes a small point at the tip to break off and cause the symptoms. The reason for the loss of the piece of chromosome is not fully understood. Jacobsen syndrome occurs more frequently in females than males. Inheritance patterns and prenatal diagnosis Inheritance patternsJacobsen syndrome occurs sporadically. For this reason, it is unusual for any other member of the family, brother or sister, to be affected. Prenatal diagnosisNone available. Is there support? Information and support in the UK for Jacobsen syndrome is provided by Unique (see entry Chromosome Disorders).