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Introduction Recent studies have shown that myosin light chain 9 (MYL9) plays a vital role in immune infiltration, tumor invasion, and metastasis; however, the prognostic and immunological role of MYL9 has not been reported. The purpose of this study was to explore the potential prognostic and immunological roles of MYL9 in human cancers by public datasets mainly including the cancer genome atlas (TCGA) and Gene expression omnibus. Methods The expression pattern and prognostic value of MYL9 were analyzed across multiple public datasets in different cancer. The correlations between MYL9 expression and immune infiltration among multiple cancers were analyzed by using the TIMER2.0. The MYL9‐related gene enrichment analysis was implemented by mainly using KEGG and GO datasets. Results MYL9 was lowly expressed in most cancers, such as breast cancer, lung adenocarcinoma and squamous cell carcinoma, and stomach adenocarcinoma; but it was highly expressed in several cancers, such as cholangiocarcinoma, head and neck squamous cell carcinoma, and liver hepatocellular carcinoma. Furthermore, MYL9 expression was distinctively associated with prognosis in adrenocortical carcinoma, colon adenocarcinoma, brain glioma, lung cancer, ovarian cancer, gastric cancer, breast cancer, blood cancer, and prostate cancer patients. The expressions of MYL9 were significantly associated with the infiltration of cancer‐associated fibroblasts, B cell, CD8+T cell, CD4+T cell, macrophage, neutrophil, dendritic cell in different tumors as well as immune markers. In addition, we found that the functional mechanisms of MYL9 involved muscle contraction and focal adhesion. Conclusion MYL9 can serve as a prognostic signature in pan‐cancer and is associated with immune infiltration. This pan‐cancer study is the first to show a relatively comprehensive understanding of the prognostic and immunological roles of MYL9 across different cancers., Our first pan‐cancer analyses of MYL9 indicated statistical correlations of MYL9 expression with clinical prognosis, immune cell infiltration across multiple tumors, which aids in understanding the role of MYL9 in tumorigenesis from the perspective of clinical tumor samples. Our results indicated that MYL9 can serve as a prognostic signature in pan‐cancer and is associated with immune infiltration.

Megacystis–microcolon–intestinal hypoperistalsis syndrome (MMIHS), or “visceral myopathy,” is a severe early onset disorder characterized by impaired muscle contractility in the bladder and intestines. Five genes are linked to MMIHS: primarily ACTG2, but also LMOD1, MYH11, MYLK, and MYL9. Here we describe a three‐year‐old girl with bilateral hydronephrosis diagnosed at 20 weeks gestation and congenital mydriasis (both of which have been previously observed among individuals with MMIHS). A clinical diagnosis of MMIHS was made based upon the presence of megacystis, lack of urinary bladder peristalsis, and intestinal pseudo‐obstruction. After initial testing of ACTG2 was negative, further sequencing and deletion/duplication testing was performed on the LMOD1, MYH11,MYLK, and MYL9 genes. We identified two heterozygous loss of function variants in MYL9: an exon 4 deletion and a nine base pair deletion that removes the canonical splicing donor site at exon 2 (NM_006097.5:c.184+2_184+10del). Parental testing confirmed these variants to be in trans in our proband. To our knowledge, only one other individual with MMIHS has biallelic mutations in MYL9 (a homozygous deletion encompassing exon 4). We suggest MYL9 be targeted on genetic testing panels for MMIHS, smooth muscle myopathies, and cardiovascular phenotypes.