Strong Genetic

Bloom Syndrome

Name: Bloom Syndrome
Creator: Realised
Published: 2026-03-01

GeneBLMrsIDrs113993962InheritanceAutosomal recessiveCarrier frequency~1 in 100 Ashkenazi Jewish, extremely rare in general populationSystemCarrier Screening

Summary

Bloom syndrome is a chromosomal instability disorder caused by deficiency of the BLM helicase — a protein critical for maintaining genomic stability during DNA replication. Without functional BLM, chromosomes undergo excessive exchanges and breaks, leading to dramatic increases in mutations throughout the body.

Practical takeaway

This matters when planning children. If your partner is also of Ashkenazi Jewish descent, the chance of them also carrying the blmAsh founder mutation is approximately 1 in 100. A carrier test through a GP or genetic counselling service can determine your partner's status. If both parents are carriers, a genetic counsellor can discuss the clinical spectrum and management of Bloom syndrome. Realised does not calculate reproductive risk or advise on reproductive decisions.

Evidence detail

What Carrier Status Means For You

You are not affected. Carriers of a single BLM variant are completely healthy — they have normal stature, normal immune function, no increased cancer risk, and normal fertility. Your carrier status becomes relevant only if your partner also carries a BLM variant. In that case, each child has a 25% chance of having Bloom syndrome, a 50% chance of being an unaffected carrier, and a 25% chance of inheriting neither variant.

Population Context

The blmAsh variant (2281del6ins7) is an Ashkenazi Jewish founder mutation that accounts for nearly all Bloom syndrome cases in this population. Outside the Ashkenazi community, Bloom syndrome is extremely rare, with sporadic cases reported across diverse ethnic backgrounds involving different BLM variants. Bloom syndrome is part of the standard Ashkenazi Jewish carrier panel recommended by ACMG and ACOG. The condition is named after dermatologist David Bloom, who first described it in 1954.

Limitations

23andMe tests 1 BLM variant — the blmAsh founder mutation. This provides good coverage for Ashkenazi Jewish individuals, where blmAsh is the predominant disease allele. For non-Ashkenazi individuals, this single variant offers very limited coverage. Over 60 pathogenic BLM variants have been identified across diverse populations. A "not detected" result on the blmAsh test alone does not meaningfully reduce carrier risk for non-Ashkenazi individuals. Comprehensive carrier screening through a clinical laboratory covers more variants and is recommended for non-Ashkenazi individuals with family history or other risk factors.

Sources (6)

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