Klinefelter syndrome (sometimes called Klinefelter's, KS or XXY) is where boys and men are born with an extra X chromosome. Chromosomes are packages of genes found in every cell in the body. There are 2 types of chromosome, called the sex chromosomes, that determine the genetic sex of a baby.SkiptomaincontentHomeHealthAtoZBacktoHealthAtoZKlinefeltersyndromeKlinefeltersyndrome(sometimescalledKlinefelter's,KSorXXY)iswhereboysandmenarebornwithanextraXchromosome.Chromosomesarepackagesofgenesfoundineverycellinthebody.Thereare2typesofchromosome,calledthesexchromosomes,thatdeterminethegeneticsexofababy.ThesearenamedeitherXorY.Usually,afemalebabyhas2Xchromosomes(XX)andamalehas1Xand1Y(XY).ButinKlinefeltersyndrome,aboyisbornwithanextracopyoftheXchromosome(XXY).TheXchromosomeisnota"female"chromosomeandispresentineveryone.ThepresenceofaYchromosomedenotesmalesex.BoysandmenwithKlinefeltersyndromearestillgeneticallymale,andoftenwillnotrealisetheyhavethisextrachromosome,butoccasionallyitcancauseproblemsthatmayrequiretreatment.Klinefeltersyndromeaffectsaround1inevery660males.Symptoms ofKlinefeltersyndromeKlinefeltersyndromedoesnotusuallycauseanyobvioussymptomsearlyinchildhood,andeventhelatersymptomsmaybedifficulttospot.Manyboysandmendonotrealisetheyhaveit.Possiblefeatures,whicharenotalwayspresent,mayinclude:in babiesandtoddlers – learning tositup,crawl,walkandtalklaterthanusual,beingquieterandmorepassivethanusualinchildhood– shynessandlowself-confidence,problemswithreading,writing,spellingandpayingattention,milddyslexia ordyspraxia,lowenergylevels,anddifficultysocialisingorexpressingfeelingsin teenagers – growingtallerthanexpectedforthefamily(withlongarmsandlegs),broadhips,poormuscletoneandslowerthanusualmusclegrowth,reducedfacialandbodyhairthatstartsgrowinglaterthanusual,asmallpenisandtesticles,andenlargedbreasts(gynaecomastia)inadulthood –inabilitytohavechildrennaturally(infertility)andalowsexdrive,inadditiontothephysicalcharacteristicsmentionedaboveHealth issuesinKlinefeltersyndr