The tan streak trait is evident within a few days of birth, since the affected deer mice are markedly paler in color than their wild-type counterparts. The phenotype is fully expressed by weaning. Superficially, the coat is white with a pale tan streak extending along the mid-dorsal line from a broader tan patch in the shoulder region. The patch with the dorsal streak forms a cross pattern in some animals. The dorsal stripe may be much abbreviated in some specimens and evident only in the cervical area, while in others it may extend to the base of the tail. Typically the streak extends about one-half to two-thirds the length of the body.
Microscopic examination of hairs from the pigmented mid-dorsal stripe shows that the stripe contains mostly hairs with sparse tan-yellow pigment granules along the length, but the pigment is somewhat more concentrated in the distal 40%. There is no distinct agouti band. However, constrictions in the individual hairs are present. The pigment has more an aspect of brown eumelanin than of phaeomelanin. Some hairs have occasional large pigment clumps. A minority of hairs in the stripe are devoid of pigment. A given hair tends to contain color granules along most of its length, or else not at all. In the lateral white portion of the coat pigment is completely absent.
The eyes are fully pigmented. The ears and tail stripe are pigmented, but are much paler and more tan than in the wild-type. The scrotum is lightly pigmented, as well.
Source: In 1986 a group of P.m. nubiterrae (Cloudland deer mouse) were captured in the southern Blue Ridge Mountains of Macon County NC, to establish a small breeding colony at Clemson University. All of the founder animals were wild-type. The following year the first tan streak deer mouse, a male, appeared from a mating between wild-caught animals.