## 4 Discussion
### How Do Oral Melanocytic Nevi Occur? Information on the etiology and pathogenesis of OMN is sparse but thought to be similar to their cutaneous counterpart [11]. At present, it is confirmed that cutaneous melanocytic nevi are in fact benign neoplasms comprising melanocytes that may harbour BRAF oncogenic mutation. It is thought that the oncogenic mutation is responsible for the initial hyperproliferation that gives rise to the nevus and the subsequent growth arrest is the result of oncogene-induced cellular senescence [46]. Considering the many similarities between cutaneous and OMN, it is plausible to conclude that the same pathogenic mechanism contributes to the development of cutaneous nevi and to the development of OMN as well. What Types Other Than Common Blue Nevi Occur in the Oral Mucosa and What Are Their Diagnostic Challenges? The literature review revealed that cellular and atypical blue nevi may also occur in the oral mucosa, though they are significantly rarer than DBN [9; 11; 34]. Unlike DBN, cellular blue nevi and atypical cellular blue nevi may pose a diagnostic challenge to pathologists [1]. CBN are histopathologically described as a well-circumscribed nodular lesion composed of lightly pigmented melanocytes. The pigmentation observed in the lesions has been described as being due to melanophages. Mitotic activity and necrosis are two features that may result in the misdiagnosis of CBN as melanoma and, therefore, it isworthwhile to remember to consider a diagnosis of an atypical cellular blue nevus rather than melanoma in the presence of such features. It is also recommended to consider the size of the lesion as a clue to differentiate ABN/CBN from melanoma, although there is controversy regarding the cut-off values with respect to size. It is the general rule to accept that as the size increases from 3 cm to 5 cm a diagnosis of melanoma becomes more likely than of ABN/CBN (34,47). The blue nevus has rarely been reported to appear in a plaque form [28]. It may present as a confluent lesion or be composed of several papules that vary from 1 cm or more. As indicated previously, the significance of the plaque-type blue nevus is its cellularity. Clinicians should be aware of the lesion as it may mimic satellite metastasis of melanoma [28]. Nevus of Ota, on the other hand, has been described to occur more often in Asian females. Age at onset shows a bimodal distribution with approximately 50% appearing at birth or in early childhood and the rest during the next four decades of life. Oral involvement is rare with only five cases reported in the literature to date [28]. A comparison of a DBN, plaque-type blue nevus and nevus of Ota reveals the following differences. Size wise, a DBN is the smallest, amounting to less than 1 cm, a plaque-type blue nevus extends to several centimetres, while the nevus Ota is the largest and more than 5 cm in size. All three lesions show a female predilection and are bluish-black in colour with the exception of the nevus of Ota, which could have a brown colour in addition to a similar colour to the other two lesions. A nevus of Ota has a higher chance of developing into a malignancy, especially in Caucasians, while the other two lesions have rarely been reported to undergo malignant transformation [28]. [PERSON] et al. [1] describe a malignant counterpart of the blue nevus. It is described as a lesion that contains collections of cytologically abnormal melanocytes and may occur in a pre-existing blue nevus, or at the site of an excised blue nevus [31]. A diagnosis of a malignant blue nevus should be excluded, especially in lesions larger than 3 cm, lesions that show a presence of a nuclear pleomorphism, atypical mitotic figures, necrosis, a destructive growth pattern and a lack of biphasic appearance [31]. Although none of the lesions described in the present series (Tables 1 and 2) showed malignant transformation, the literature review revealed a malignant transformation of a blue nevus on the upper lip, which occurred in a 50-year-old man.