Class I microdeletion (BP3-BP4)

Clinical description

Class I 1q21.1 microdeletion syndrome has variable expressivity and incomplete penetrance. This means that a carrier of this genetic change can have no manifestations and those could vary between people.

Associated malformations

  • Physical features
  • Carrier of this microdeletion can prensent some particular physical features. Principally, 50% of cases present a microcephaly, namely a smaller head. Also, some specific facial traits can be observed (frontal bossing, deep-set eyes, bulbous nose). A small stature is observed in half of patients.

  • Ocular abnormalities
  • About 26% of patients present ocular abnormalities like strabismus, hypermetropia, cataracts, and smaller eyes (microphtalmia).

  • Cognitive delay
  • About 30% of people carrying this deletion present a mild intellectual deficit. Therefore, the majority don’t have a cognitive delay.

  • Developmental delay
  • Majority of children carriers of the distal 1q21.1 microdeletion present a developmental delay. In fact, about 75% of children have a speech or motor delay.

  • Psychiatric and behavioral troubles
  • Between others, carriers can present an autism spectrum disorder, an attention deficit hyperactivity disorder (10%), a schizoprenia, and sleep disturbance.

  • Neurological troubles
  • About 15% of patients present a seizure disorder.

  • Malformations
    • Cardiac
    • Genitourinal
    • Skeletal


    • Transmission: Microdeletions are transmitted on an autosomal dominant mode. A carrier has a probability of 50% to transmit this deletion to her descents.
    • In 18-50% of cases, the microdeletion happened de novo, which means during the conception of an affected person. The risk for her brothers and sisters is in this case similar to the population risk.
    • The microdeletion can also be inherited from a parent having similar and milder symptoms to his child or can by asymptomatic. The risk for brothers and sisters is then of 50% to inherit the microdeletion. It is therefore no possible to predict the manifestations.
    • Dépistage prénatal: Le dépistage prénatal est techniquement réalisable soit par la biopsie des villosités choriales à partir de 11 semaines de grossesse ou l’amniocentèse à partir de 15 semaines de grossesse. Peu de demandes ont cependant été faites pour cette condition vu la variabilité clinique associée et l’impossibilité de prédire les manifestations.


    The 1q21.1 reciprocal microdeletion has been associated to an increased cerebral size (macrocephaly) which is the contrary of what is observed in the microdeletion.

    Notes that little is know about the disease and this is why it's very important for studies to understand how this region can cause difficulties for some individuals carrying a certain rearrangement, and save others with the opposite one.

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