The reduced pigmentation (rp) mutation causes defects in organelle biogenesis. On a non-agouti background, mice homozygous for the recessive rp mutation have lighter coat color, pale ears and tail, and the eyes are dark. This coat color is similar in intensity to that caused by the cocoa or ruby eye-2 mutations, and there is no change in coloration with age. Homozygotes are viable and fertile, but have an increased bleed time, decreased splenic NK cell activity, and the melanosomes fail to properly mature (Gibb et al., 1981; Ahmed et al., 1989). There are fewer ellipsoidal type IV melanosomes, and the striated melanosomes are elongated but irregularly shaped indicating that rp disrupts the melanosomal maturation step II (Nguyen et al., 2002). These pigment granules are smaller than normal, but they are not found clumped together. Increased beta-galactosidase, beta-glucuronidase, and N-acetyl-beta-hexosaminidase activities are found in kidney homogenates of rp homozygotes similar to the increases found in these lysosomal glycosidase activities in a group of Hermansky-Pudlak model mice including those homozygous for Lystbg, Vps33abf, Hps1ep, Hps4le, Bloc1s6pa, Ap3b1pe, or Hps6ru. rp reduces the incidence of splenic lipofuscinosis on a C57BL background indicating that the lysosomes of macrophages are impacted (Ahmed and Shire, 1985). As with mice homozygous for Lystbg, rp homozygotes are more susceptible to the anaesthetics pentobarbital, tribromoethanol, and alphaxalone. (Ahmed et al., 1989).
The reduced pigmentation mutation arose spontaneously on the C57BL/Tb background and was backcrossed twice to the C57BL/10ScSn background. In 1989 C57BL-rp mice were sent from Dr. John Shire at The University of Essex to Dr. Ben Taylor at The Jackson Laboratory where the strain was maintained by sibling mating primarily homozygous females with heterozygous males. In 1994 C57BL/6J females were bred with F19 homozygous males to generate embryos for cryopreservation.