Hyper-IgM syndrome due to UNG deficiency is a very rare entity with isotype switching deficiency (<1%) to date, there are very few cases of UNG deficiency, it participates in isotype switching and somatic hypermutation of immunoglobulin loci.

Female patient from Querétaro, hereditary family history of consanguinity. His condition began from the first year of life with recurrent upper respiratory infections every week, uncomplicated, accompanied by persistent fever, productive cough.

At 5 years, he presented a left cervical abscess with surgical drainage and a positive culture for Streptococcus pneumoniae, with right cervical adenopathy, a lymph node biopsy was performed with a histopathological report showing mixed lymphoid hyperplasia suggestive of viral lymphadenitis.
Physical examination: oropharynx with hypertrophy of grade II tonsils, with bilateral cervical adenopathies of 1 cm soft, bilateral inguinal nodes smaller than 0.5 cm.
IgG 18, (633-1289) IgM 448.7 (48-207), IgA 0, (33-202) IgE 0.10 00 (1.03-161.3 IU/ mL ).
The approach for inborn error of immunity hyper-IgM syndrome is started.
No alterations in the expression of HIGM-related proteins such as AID (activation-induced deaminase) and CD40 were detected.
Exome sequencing was performed, detecting a homozygous missense variant in exon 6 of UNG, p.N213S. This is as highly conserved residue in evolution; it is located in the catalytic domain of the enzyme. This is a variant previously reported as a polymorphism with an allelic frequency less than 0.1. However, it is predicted to be a pathogenic variant as Polyphen-2 indicates a score of 1.0 for this variant and a CADD score of 24.

This would be the first case of UNG deficiency described in Mexico and the fifth pathogenic variant described in the world, it has been described that patients are susceptible to developing lymphoma, the patient described here is currently 15 years old, with good evolution, with replacement gammaglobulin.