There are transcription factors (TFs) or co-regulators (CRs) that don't clearly belong to any of the other families, thus they are called orphans. They often possess one or more of the requirements of belonging to a TF factor or cor-regulator family but not the complete requirements.  They may represent incomplete gene models or pseudogenes. As related TFs and CRs are identified and genome revisions  provide improved gene models, they may be assigned to existing or new families.

Although Orphan genes are relatively unstudied they may represent undervalued genes with possible agronomic implications (Jiang et al., 2022, Arendsee et al., 2014, Tanvir et al., 2022)..

Last updated June 2023 by John Gray

References:

Jiang ML, Lang H, Li XN, Zu Y, Zhao J, Peng SL, Liu Z, Zhan ZX, Piao ZY. Progress on plant orphan genes. Yi Chuan. 2022 Aug 20;44(8):682-694. doi: 10.16288/j.yczz.22-154. PMID: 36384667.

Arendsee ZW, Li L, Wurtele ES. Coming of age: orphan genes in plants. Trends Plant Sci. 2014 Nov;19(11):698-708. doi: 10.1016/j.tplants.2014.07.003. PMID: 25151064.

Tanvir R, Wang L, Zhang A, Li L. Orphan Genes in Crop Improvement: Enhancing Potato Tuber Protein without Impacting Yield. Plants (Basel). 2022 Nov 13;11(22):3076. doi: 10.3390/plants11223076. PMID: 36432805; PMCID: PMC9696052.