Clinical Genetics

The Genetics of 1q21.1: Distal, Proximal, and TAR Syndrome

At a Glance

1q21.1 microdeletions differ by the chromosome segment that is missing, so distal, proximal, and larger deletions can have different effects. A proximal deletion alone does not diagnose TAR syndrome; testing the remaining RBM8A gene and reviewing limb and platelet findings may be needed.

The name “1q21.1 microdeletion” is actually an umbrella term for several different genetic “neighborhoods” on chromosome 1. Because these neighborhoods contain different genes, where the deletion starts and ends determines the specific health and developmental risks you might face ^[1]^[2].

Distal vs. Proximal: Two Different Neighborhoods

Think of chromosome 1 as a long road with different landmarks called breakpoints (BP). The 1q21.1 region is generally divided into two primary sections:

1. The Distal Region

Often described by laboratories as falling between breakpoints BP3 and BP4, this is the “classic” recurrent 1q21.1 microdeletion region. It is about 1.35 Megabases (Mb) in size and contains a set of genes that have been studied as possible contributors to brain growth and heart development ^[2]^[3].

CHD1L: This gene has been studied as a possible contributor to brain development and body growth. Research shows that having too little of this gene (a deletion) is linked to smaller head size (microcephaly), while having an extra copy (a duplication) is linked to larger head size ^[4]^[5].
NOTCH2NL: This gene has been studied as a possible contributor to regulating how many neurons the brain produces, which may relate to the smaller head size and learning challenges often seen in this syndrome ^[6]^[5].
GJA5: This gene has been studied as a potential contributor to the structural and electrical development of the heart ^[7].

2. The Proximal Region

Often described as falling between breakpoints BP2 and BP3 (though some labs label it BP1-BP2), this region contains the RBM8A gene ^[8]^[9]. This region is evaluated for its association with TAR syndrome, but a deletion here does not by itself cause the syndrome ^[10].

3. Class II Deletions

In some cases, a person is missing a very large piece of DNA that stretches across both regions, from BP1 all the way to BP4, or involves atypical breakpoints ^[8]^[1].

The Truth About TAR Syndrome

One of the most common misunderstandings upon diagnosis is the assumption that a proximal 1q21.1 deletion means a child has TAR (Thrombocytopenia-Absent Radius) syndrome.

A proximal deletion does not by itself cause TAR syndrome. TAR syndrome follows a “two-hit” or compound inheritance model ^[11]^[12]:

Hit 1: A deletion of the proximal 1q21.1 region (missing one copy of the RBM8A gene).
Hit 2: An additional variant (such as a sequence or regulatory change) on the remaining RBM8A gene on the other chromosome 1, which a standard microarray might not detect ^[13]^[14].

A diagnosis requires the appropriate RBM8A deletion/variant combination, evaluated alongside clinical signs like limb differences and platelet history ^[10]^[15]. If a proximal deletion is found, a genetics professional will lead a review of the interval and recommend targeted RBM8A testing and clinical evaluation.

Inheritance: Where Did It Come From?

When a 1q21.1 deletion is found, one of the first questions is whether it was inherited or occurred by chance.

De Novo: Estimates vary depending on the clinic, but many cases are “de novo” (new events), meaning neither parent carries the deletion ^[2]. If parental blood tests are negative, recurrence risk is usually low (though not absolutely zero due to possible germline mosaicism).
Inherited: In other cases, the deletion is passed down from a parent ^[2]. If a parent carries the deletion, they generally have a 50% chance of passing it on in each pregnancy.

Because of variable expressivity and incomplete penetrance, a parent can carry the deletion and be clinically unaffected, yet pass it to a child who faces challenges ^[3]^[16]. Testing siblings or extended relatives should be arranged through genetic counseling, based on consent and whether the result would meaningfully change medical care. Knowing if a parent carries the deletion helps doctors understand reproductive risks and ensures the parent receives baseline health screenings, such as a cardiovascular assessment ^[3]^[17].

Common questions in this guide

What is the difference between a distal and proximal 1q21.1 deletion?

The distal deletion usually lies between chromosome landmarks called BP3 and BP4 and is the classic recurrent region, measuring about 1.35 megabases. The proximal deletion is usually between BP2 and BP3, although some laboratories use BP1 and BP2, and it includes the RBM8A gene. Because different genes are missing, the possible health concerns and follow-up can differ.

Does a proximal 1q21.1 deletion automatically mean TAR syndrome?

No. A proximal 1q21.1 deletion is only one part of the genetic pattern linked with TAR syndrome. TAR syndrome requires an additional sequence or regulatory change in the remaining RBM8A gene, along with compatible findings such as limb differences and a history of low platelets. A standard microarray may not detect that second change.

What testing is needed if a proximal 1q21.1 deletion is found?

A genetics professional should confirm which chromosome interval is deleted and review the clinical signs. Targeted testing of the remaining RBM8A gene may be recommended, including sequence or regulatory testing, because a standard microarray may miss this type of change. The evaluation may also include a review of limb findings and platelet history.

Can a parent pass a 1q21.1 deletion to a child?

Yes. The deletion may be new in a child, or it may be inherited from a parent. If a parent carries it, the chance of passing it on is 50% in each pregnancy; if parental blood tests are negative, the chance is usually low but not zero because a change can rarely be limited to reproductive cells.

Can someone carry a 1q21.1 deletion without noticeable symptoms?

Yes. Because of variable expressivity and incomplete penetrance, a parent can carry the deletion with few or no noticeable effects while a child has developmental or medical challenges. A parent who carries the deletion may also be offered a baseline cardiovascular assessment.

Should siblings or other relatives be tested for a 1q21.1 deletion?

Possibly, but testing should be planned with a genetic counselor. The decision should consider the person's consent, age, and whether the result would change medical care. If a parent carries the deletion, family results can clarify reproductive chances and help identify appropriate baseline screening.

Get notified when new evidence is published on 1q21.1 microdeletion syndrome.

We monitor PubMed for new peer-reviewed studies on this topic and email a short summary when something meaningful changes.

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This page is for informational purposes only and does not constitute medical advice. A clinical geneticist or genetic counselor should interpret an individual 1q21.1 result and guide RBM8A testing, family testing, and cardiovascular screening.